To keep the unchanged number of grooves coming in contact with a ball even at a low clubhead speed, compared with the number at a high clubhead speed, by changing the breadth and intervals of the grooves at every clubhead speed, and maintain optimum groove intervals and breadth for the cover material of a golf ball.

The breadth (t) and intervals L of grooves 1 formed on a clubhead are respectively set approximately at 0.9mm and at a value three to five times as large as the breadth (t) for a club capable of giving a longest carry, or the smallest club number. In addition, the breadth (t) and the intervals L are set so as to decrease gradually or in a step according to an increase in the club number, and respectively set approximately at 0.1mm and 1.9mm for the largest club number.