To improve the workability of the attaching/detaching operation of the blade in a blade attaching/detaching device, which also serves as a slack prevention mechanism for the blade, with a simple appearance so that the cutting state of the blade can be easily confirmed in the operation.

A female screw 7 is provided at the outer circumference of one end portion of a plunger 1, and a blade stopping part 2 for accepting the shoulder part of a blade so as to support the blade in the axial direction of the plunger 1 is provided inside the plunger 1. Further, a through long hole 9 elongating in the axial direction of the plunger 1 is provided in the plunger 1 where the female screw 7 is provided. A fixing member 4 having a ring groove 10, to be fitted with the female screw 7 provided at the outer periphery of the plunger 1 by screwing, and a hole communicating with the ring groove 10 of the fixing member 4 is provided inside the plunger 1 so as to be fixed with a pin 5 with both end parts sufficiently accommodated in the ring groove 1 of the fixing member 4 via the through long hole 9 of the plunger 1, and a pressing member 3 always forced to the blade stopping part 2 side by a coil spring 6 is provided.