PURPOSE: To easily and manually adjust the clamping force in an on-energized condition without using any tool in a clamping device, particularly, a clamping force adjusting device for a machine vice.

CONSTITUTION: This clamping force adjusting device is provided with a hollow housing 1 supported by a movable clamp part of a clamping device, a first end part of a rotatable and slidable tension rod 5 piercing the housing is connected to a force amplifier, and a second end part 5b carries a tension sleeve 6 fixed to the end part. The tension sleeve 6 is supported by Belleville springs 9 through a thrust bearing 8 and an intermediate sleeve 10, and the Belleville springs 9 is directly supported in the housing 1. An adjustment sleeve 13 is screwed in a fine thread 14 of the housing 1, a first stop shoulder 16 inwardly in the radial direction is provided on an end part opposite to the Belleville springs 9, and the first stop shoulder 16 cooperates with a second stop shoulder 17 outwardly in the radial direction of the intermediate sleeve 10, and a spacer ring 18 to be attached to the thrust bearing 8.