Golf club head

1. A hollow golf club head comprising:

a face portion having a front face that defines a club face for hitting a ball;

a crown portion intersecting the club face at the upper edge thereof; and

a sole portion intersecting the club face at the lower edge thereof, wherein

the crown portion is provided along the front edge thereof with a face-backing zone in which the rigidity is gradually increased from its toe-side end and heel-side end toward the center thereof by changing the thickness in the face-backing zone such that the thickness is gradually increased from the toe-side end and heel-side end toward the center thereof, and the maximum thickness t 1 occurring in the increased-thickness central part is in a range of not less than 0.6 mm and not more than 2.0 mm, and a minimum thickness t 2 and a minimum thickness t 3 occurring on the toe-side and the heel-side, respectively, of the increased-thickness central part are in a range of not less than 0.5 mm and not more-than 1.5 mm.

2. A hollow golf club head comprising:

a face portion having a front face that defines a club face for hitting a ball;

a crown portion intersecting the club face at the upper edge thereof; and

a sole portion intersecting the club face at the lower edge thereof, wherein

the sole portion is provided along the front edge thereof with a face-backing zone in which the rigidity is gradually increased from its toe-side end and heel-side end toward the center thereof by changing the thickness in the face-backing zone such that the thickness is gradually increased from the toe-side end and heel-side end toward the center thereof, and the maximum thickness ti occurring in the increased-thickness central part is in a range of not less than 0.8 mm and not more than 2.0 mm, and a minimum thickness t 2 and a minimum thickness t 3 occurring on the toe-side and the heel-side, respectively, of the increased-thickness central part are in a range of not less than 0.6 mm and not more than 1.8 mm.

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

the ratio (t 1 /t 2 ) of the maximum thickness t 1 and the thickness t 2 is in a range of from 1.2 to 3.0, and

the ratio (t 1 /t 3 ) of the maximum thickness t 1 and the thickness t 3 is in a range of from 1.2 to 3.0.

4. The hollow golf club head according to claim 3 , wherein

a width W 1 of said central part in the toe-heel direction is more than 10 mm.

5. The hollow golf club head according to claim 3 , wherein

a width W 1 of said central part in the toe-heel direction is more than 15 mm.

6. The hollow golf club head according to claim 3 , wherein

a width W 1 of said central part in the toe-heel direction is more than 20 mm.

7. The hollow golf club head according to claim 3 , wherein

a width W 1 of said central part in the toe-heel direction is more than 25 mm.

8. The hollow golf club head according to claim 3 , wherein

a width W 1 of said central part in the toe-heel direction is more than 30 mm.

9. The hollow golf club head according to claim 3 , wherein

the face-backing zone has a depth L 1 in a range of from 5 to 50% of the depth L of the club head.

10. The hollow golf club head according to claim 1 , wherein

the crown portion has a convexly curved even outer surface and an inner surface.

11. The hollow golf club head according to claim 10 , wherein

said inner surface has, in cross section parallel to the toe-heel direction, a contour made up of a central convex curve and a concave curve on each side thereof, having at least two steps, being straight, having two straight lines intersecting each other at an angle slightly smaller than 180 degrees, or made up of a concave curve.