To positively prevent excessive fitting of a bit 2 by effectively using a base face 7a of a fitting hole 7.

The bit 2 fitted into a chuck part 3 provided at a rotating tool 1 such as an electric rotating tool and formed with a bit part 2a at the end is locked to the chuck part 3 in a prescribed distance position from the bit part 2a. The chuck part 3 has the fitting hole 7 for fitting the bit 2, and the base part side of the fitting hole 7 is provided with the base face 7a. The base face 7a is provided with a recessed part 4 fitted with the bit part 2 therein and extended in the lengthwise direction of the fitting hole 7. The bit 2 is further provided with abutting parts 5 abutting on the base face 7a, in the end side outer peripheral position.