Abstract: A screw-operated clamping mechanism comprises a clamp body, a shaft attached to a side of the clamp body, a movable pawl which is slidably attached to the shaft, a knob which is allowed to slide and turn relative to the shaft, an I-shaped nut which is rotatably screwed onto a threaded portion formed in the shaft, and a stopper attached to an end of the shaft. The knob is provided, at a side facing the I-shaped nut, with a first contact face with which the I-shaped nut comes in contact, a concave nut receiving groove formed in the first contact face, a second contact face which is formed at the bottom of the nut receiving groove and with which the I-shaped nut comes in contact, and nut locking parts which project from the first contact face and set the knob and the I-shaped nut to be turned integrally.