Golf club head

1. A golf club head comprising

a main body and

a crown cover disposed in a crown portion of the head,

said main body comprising a crown base made of a metal material and provided with a plurality of openings,

said openings gradually decreasing in the opening area from the backside to the foreside of the head, and

said crown cover made of a material whose specific gravity is lower than said metal material, and disposed on the crown base so as to cover said openings, wherein said openings are

a single first opening formed in the rear of the crown base, and

a plurality of second openings smaller than the first opening and formed on the front side of the first opening,

the second openings each have an area of from 0.5 to 2.0 sq.cm,

the number of the second openings is in a range of from 10 to 20,

the first opening has an area of from 20 to 30 sq.cm, and

the centroid of the first opening is positioned on the backside of a position which is at a distance of 66% of the depth (CL) of the crown portion backwardly from the front end of the crown portion.

2. The golf club head according to claim 1 , wherein the second openings have substantially same opening areas.

3. The golf club head according to claim 1 , wherein the second openings are the substantially same shapes having substantially same opening areas.

4. The golf club head according to claim 2 or 3 , wherein the total area S 2 a of the second openings is in a range of from 0.3 to 1.0 times the area S 1 of the flrst opening.

5. A golf club head comprising

a main body and

a crown cover disposed in a crown portion of the head,

said main body comprising a crown base made of a metal material and provided with a plurality of openings,

said openings gradually decreasing in the opening area from the backside to the foreside of the head, and

said crown cover made of a material whose specific gravity is lower than said metal material, and disposed on the crown base so as to cover said openings, wherein

said openings are a single first opening formed in the rear of the crown base, and a plurality of second openings smaller than the first opening and formed on the front side of the first opening,

the number of the second openings is in a range of from 10 to 20,

the second openings each have an area of from 0.5 to 2.0 sq.cm,

the first opening has an area of from 20 to 30 sq.cm, and

the first opening is elongated in a toe-heel direction of the head and curved backward of the head so that the first opening has a front edge and a rear edge which are curved backward of the head, and the second openings are positioned on the front side of said front edge.

6. The golf club head according to claim 5 , wherein

the total area S 2 a of the second openings is in a range of from 0.3 to 1.0 times the area S 1 of the first opening.

7. The golf club head according to claim 5 , wherein the second openings have substantially same opening areas.

8. The golf club head according to claim 5 , wherein the second openings are the substantially same shapes having substantially same opening areas.

9. The golf club head according to claim 1 or 5 , wherein a front end zone of the crown portion defined as having a depth of from 8 to 25 mm is not provided with any opening.

10. The golf club head according to claim 1 or 5 , wherein

the second openings are arranged in a plurality of rows each extending in the toe-and-heel direction, and

the number of the opening or openings in each of the rows is increased from the backside to the foreside of the head, and then decreased.

11. The golf club head according to claim 10 , wherein said number is 1 , 2 , 3 , 4 and 3 , respectively, from the backside to the foreside of the head.