PURPOSE: To enable precise positioning as a spherical surface of a ball touches a space even if the ball is not formed precisely according to a pitch of a tooth profile, by performing positioning clamp by holding the ball, fitted in the tip part of a shaft part with play, to the space.

CONSTITUTION: A concave hole 8a for gear housing is formed on the center of a gear casing 8 of a gear chucking device and three pin insertion holes 9 protruded in a diametral direction are provided at trisectional intervals on the side of the holes 9 so as to fit a space of a gear. Gear pin 10 is inserted into the pin insertion hole 9 and is held within the insertion hole 9 by clip 11 penetrating through by meeting at right angles with an insertion direction of the pin 10. The tip of the gear pin 10 is protruded into the space of the gear W and the tip is provided with a ball 104. The ball 104 clamps the gear W while the ball 104 is being rotated and touched to one side of the space by making free movement in a direction meeting at right angles with an axial direction.