PURPOSE: To make a torque detecting mechanism simple by making a plunger freely movable into and out from the inside of the pressure chamber of an oil hydraulic circuit, and converting the pressing force of a keep plate which fixes the plunger to oil pressure.

CONSTITUTION: When a prime mover 2 is operated, the revolutions of the prime mover 2 are transmitted to a driven machine 3 by means of a torque detector 10 serving also as a claw coupling. Since a keep plate 19 is mounted between the claw parts 15 and 17 of the claw coupling, the revolutions are transmitted with the plate 19. A plunger is fixed to the plate 19 and the plunger is provided in the pressure chamber filled with oil; therefore pressure is generated in oil lines 21, 23, 24 by the pressing force 19 acting upon the plate 19. The generated pressure is read with an oil pressure gage 25. Since the pressure indicated on the gage 25 is proportional to torque, the gage 25 is usable as a torque indicator.