To replace a saw blade easily and rapidly with a moving direction facilitating the manual operation of a worker by engaging the saw blade with a tightening sleeve in the state of being taken into a release position from the tightening sleeve only in one operating direction, and automatically releasing engagement with the tightening sleeve.

A trigger 29 is formed longer than the possible stroke of an elevating rod 18 and brought into contact with a cam 21' at the time of turning motion. The cam 21' is supported on a tightening sleeve 21, radially over the peripheral part of the tightening sleeve 21. When a key 28 is operated, the trigger 29 moves passing close to the tightening sleeve 21, and at this time, takes the cam 21' along. The turning motion of the trigger 29 is thereby switched into the rotary motion of the tightening sleeve 21. In the final position of rotation, a tightening end part 22 is released, and a saw blade can be taken out in an axial direction.