To enable a structure to easily mount a bit by light force, also after once mounting, prevent the bit from being unexpectedly come off, and facilitate extracting of the bit, in the bit holding structure for a driver.

This structure comprises an inner cylinder body 5 having a bit insertion hole 4 opened to a tip end side, an outer cylinder body 6 externally fitted to be slidably in a direction of axial center L to the inner cylinder body 5, elastic repulsive member 7 arranged between inner/outer cylinder bodies 5, 6, and a ball body 9 loosely fitted to a diametric direction hole part 8 provided to be passed through in a peripheral wall part of the inner cylinder body 5. In an internal peripheral surface of the outer cylinder body 6, an inner protrusive strap 20 receiving the ball body 9 is formed, the inner protrusive strap 20 has an assigned internal peripheral surface 23 in parallel to the axial center L in a vertical sectional shape and a bit insertion guide slope 24 continued diagonally outward from a base end of the assigned internal peripheral surface 23.