Connection of a bit with two balls is enabled while high operability of attachment and detachment of the bit is maintained. A pressing ring 23 is held at the advance position together with an operation sleeve 22 by a guide washer 24 forced by a coil spring 25. When the rear end of the bit is inserted into a tool holder 8, the balls 27, 27 that are rolled rearward by the contact of the rear end of the bit are pushed in between the pressing ring 23 and the guide washer 24 in order, and the balls 27, 27 can be retracted from the tool holder 8 in the state where the pressing ring 23 is held at the advance position by the tilt of the guide washer 24.