To prevent deterioration of power transmission belt and drop of power transmission efficiency, inhibit cost increase, and save a space, in a construction enabling regular and reverse rotation change-over of a cooling fan.

This mechanism has a regular rotation and a reverse rotation fan pulleys 4, 5 rotatably installed on a shaft of a cooling fan 1, and is provided with a regular rotation power transmission belt 9 transmitting rotation of an output shaft 8 of the engine to the regular rotation fan pulley 4, an idler pulley 12 pressed and made contact on an outside of the regular rotation power transmission belt 9 and rotating in a direction reverse to a drive pulley 7, a reverse rotation pulley 14 rotating as one unit with the idler pulley 12, and a reverse rotation power transmission belt 15 transmitting rotation of the reverse rotation pulley 14 to the reverse rotation fan pulley 5. The mechanism is provided with a clutch 6 transmitting rotation of either of the regular rotation and the reverse rotation fan pulleys 4, 5 to a shaft 2 of the cooling fan 1.