To reliably correct tensile force by controlling a working fluid between a damper chamber and a reservoir, instantly allowing idler propelling motion by two kinds of energizing means at tensile force reducing time of a belt-like motive power transmitting member, and gradually allowing retreat motion at tensile force increasing time.

When tensional force of a timing belt becomes a preset value or less, a spring 60 push-moves the belt through an idler, and a damper chamber R1 is put under negative pressure by projecting operation of a piston 40 by a spring 43 having energizing force weaker than the spring 60, and a ball valve 41 is opened, and a working fluid flows in the damper chamber R1 from a reservoir chamber. When the tensional force of the belt exceeds the preset value, the belt push-moves the idler, and pressure in the damper chamber R1 increases by intrusion of the piston 40 to close the ball valve 41, and the working fluid in the damper chamber R1 flows in the reservoir through a fitting clearance between the piston 40 and a cylinder inside hole 31a and a communicating hole of the piston 40. Therefore, a movement of the idler becomes quick for looseness of the belt, and becomes slow for tension, precisely adjusting the tensional force of the belt can.