A driving force transmitting devices is able to engage a main clutch with a small amount of hydraulic pressure. The driving force transmitting device includes an input shaft and an output shaft, and a main clutch. The device is formed of a primary clutch that intermittently transmits the driving force transmitted from the input shaft, a hydraulic pump that generates a hydraulic pressure, a piston that presses the primary clutch with the hydraulic pressure by the hydraulic pump, and a cam mechanism that presses the main clutch with a press-contact force which is amplified to be larger than the press-contact force of the piston by utilizing the driving force from the input shaft through the primary clutch, so as to engage the main clutch.