A power transmission device is proposed which makes it easy to start on a slope. A control type slip clutch and a mechanical type control type two-way clutch are mounted between an input shaft to which the rotation of an engine is transmitted and an output shaft mounted around the input shaft. The input shaft and the output shaft are engaged together by the slip clutch while producing slip, and thereafter the two-way clutch is engaged. This improves transmission efficiency. A one-way clutch is provided between the output shaft and a housing provided around the output shaft. In a stopped state on an upward slope, when depressing of the brake pedal is released and rotation in the reverse direction to the revolving direction of the engine is transmitted to the output shaft from the vehicle wheel side, the output shaft is locked by the operation of the one-way clutch, thereby preventing the vehicle from moving in the opposite direction to the moving direction.