PURPOSE: To provide a structure wherein a reaction force to a fastening force is exerted as a compression force on a frame, in a vice for a machine tool having a mechanical step-up means.

CONSTITUTION: A fixed mouthpiece 13 is fixed to one end of a frame 11. A moving mouthpiece 14 is guided by a frame and joined in a screwed-in manner with a screw shaft 12, and approaches and parts from the fixed mouthpiece. A pressure receiving member 17 is arranged coaxially with the screw shaft. A mechanical step-up means 15 is incorporated in the hollow part of a screw shaft. The one end of the reaction force receiving member is coupled to the mechanical step-up means, and axial movement of the other end to the frame is regulated in the vicinity of the fixed mouthpiece. Through operation of the mechanical step-up means, micromovement of the screw shaft in an axial direction is carried out to firmly fasten a work between the fixed and moving mouthpieces. The reaction force of a fastening force is exerted on the frame end face on the fixed mouthpiece side through the reaction force receiving member.