To enable a thumb holder in which a hole for inserting the thumb is formed to be removably mounted in a thumb-solid hole in a bowling ball.

A locking member 20 having a positioning projection 20b is bonded to the bottom of a thumb-solid hole formed in a ball, and a thumb solid 10 in which a hole 4 for inserting the thumb is formed is inserted into the thumb solid hole from above the locking member 20 as it is aligned with the thumb solid hole, and a locking bolt fitted into a nut provided at the locking member 20 is tightened to fix a thumb holder in place. Removal is effected by drawing out the thumb solid 10 through screw-and-jack action produced when a withdrawal bolt fitted into a nut provided at the bottom plate part 10b of the thumb solid is pressed against the top surface plate 21 of the locking member 20.