Golf club head

1. A golf club head comprising:

a face portion having a front face forming a club face and a rear face facing a hollow;

a sole portion extending backward from the face portion and having an inner surface facing the hollow and an outer surface forming an undersurface of the head, the sole portion provided with a tubular part having a hole that forms a socket; and

a weight member secured in the socket,

wherein

the tubular part protrudes from the inner surface of the sole portion into the hollow,

the sole portion is provided on the outer surface with at least one stiffening groove so that said at least one stiffening groove forms at least one stiffening rib on the inner surface of the sole portion, and

the stiffening groove is partially included in a vicinity zone which is defined as extending 10 mm from the socket,

wherein

said at least one stiffening groove is a toe-side groove and a heel-side groove each not connected to the socket,

the heel-side groove extends towards a heel side from a starting point within the vicinity zone while curving towards a club face side, and

the toe-side groove extends towards a toe side from a starting point within the vicinity zone while curving towards a club face side.

2. A golf club head comprising:

a face portion having a front face forming a club face and a rear face facing a hollow;

a sole portion extending backward from the face portion and having an inner surface facing the hollow and an outer surface forming an undersurface of the head,

the sole portion provided with a tubular part having a hole that forms a socket; and

a weight member secured in the socket,

wherein

the tubular part protrudes from the inner surface of the sole portion into the hollow,

the sole portion is provided on the outer surface with at least one stiffening groove so that said at least one stiffening groove forms at least one stiffening rib on the inner surface of the sole portion, and

the stiffening groove is partially included in a vicinity zone which is defined as extending 10 mm from the socket, wherein

said at least one stiffening groove is a toe-side groove and a heel-side groove each connected to the socket,

the heel-side groove extends towards a heel side from the socket while curving towards a club face side, and

the toe-side groove extends towards a toe side from the socket while curving towards a club face side.

3. A golf club head comprising:

a face portion having a front face forming a club face and a rear face facing a hollow;

a sole portion extending backward from the face portion and having an inner surface facing the hollow and an outer surface forming an undersurface of the head,

the sole portion provided with a tubular part having a hole that forms a socket; and

a weight member secured in the socket,

wherein

the tubular part protrudes from the inner surface of the sole portion into the hollow,

the sole portion is provided on the outer surface with at least one stiffening groove so that said at least one stiffening groove forms at least one stiffening rib on the inner surface of the sole portion, and

the stiffening groove is partially included in a vicinity zone which is defined as extending 10 mm from the socket, wherein

said at least one stiffening groove is

a groove extending straight towards the club face from a starting point within the vicinity zone, and

a groove extending straight towards the back side of the head from a starting point within the vicinity zone.

4. A golf club head comprising:

a face portion having a front face forming a club face and a rear face facing a hollow;

a sole portion extending backward from the face portion and having an inner surface facing the hollow and an outer surface forming an undersurface of the head,

the sole portion provided with a tubular part having a hole that forms a socket; and

a weight member secured in the socket,

wherein

the tubular part protrudes from the inner surface of the sole portion into the hollow,

the sole portion is provided on the outer surface with at least one stiffening groove so that said at least one stiffening groove forms at least one stiffening rib on the inner surface of the sole portion, and

the stiffening groove is partially included in a vicinity zone which is defined as extending 10 mm from the socket, wherein

said at least one stiffening groove is a single groove extending straight in the heel-and-toe direction of the head, and

the minimum distance between the stiffening groove and the socket is in a range of from 1.0 to 10.0 mm and occurs in the middle of the length of the stiffening groove.

5. The golf club head according to claim 1 , 2 , 3 or 4 , wherein

the socket provided in the sole portion is single.

6. The golf club head according to claim 1 , 2 , 3 or 4 , wherein

the socket provided in the sole portion is single, and formed at a position in a range of not less than 0.5 times the length from the front end to the rear end of the head in the bottom plan view of the head from the front end.

7. The golf club head according to claim 1 , 2 , 3 or 4 , wherein

said at least one stiffening groove has a depth of from 0.5 to 2.0 mm.

8. The golf club head according to claim 1 , 2 , 3 or 4 , wherein

the thickness of the wall forming said at least one stiffening groove is not more than 2.0 mm, and not less than the thickness of a main part of the sole portion.

9. The golf club head according to claim 1 , 2 , 3 or 4 , wherein

each said stiffening groove has a length of not less than 15 mm and an opening width of from 1 to 10 mm.

10. The golf club head according to claim 1 , 2 , 3 or 4 , wherein

the socket comprises: a threaded inner part having a first inside diameter at the top of the thread; and an outer part having a second inside diameter larger than the first inside diameter, and

the weight member comprises: a threaded part engaging with the threaded part of the socket; and an increased-diameter part placed within the outer part of the socket.