To provide a power transmitting mechanism of simple structure at a low cost having reliance upon shutting-off of the power exceeding the set value.

Three pairs of plate spring mounting projections 1b are provided at a constant spacing in the inside circumferential surface of the rim 1a of a pulley 1. Each pair of projections 1b are furnished with a triangular cutout 1c opposingly. Three curved recesses 2c are formed at a constant spacing at the peripheral surface 2b of the rim 2a of a hub 2. Three plate springs 7 are deflected, and the ends 7b of each plate spring 7 are engaged with the cutout, and the central part 7a is put in pressure contact with respective curved recess. When a torque exceeding the set value is applied to the hub 2, the central part of each plate spring is pressed to the peripheral surface of the rim of hub 2 (portion where no curved recess is formed), so that the deflecting direction of each plate spring 7 is inverted, As a result, the plate springs 7 do not touch the hub 2, and no power is transmitted from the pulley 1 to the hub 2.