Provided is a power transmission device that lets a motor in a hybrid vehicle be smaller and lighter, prevents internal resistance in the motor and inertia from becoming driving resistance, and can effectively utilize energy. Via a first clutch (10), power is transmitted from an input shaft (2) to a transmission shaft (3) but power transmission from the transmission shaft (3) to the input shaft (2) is blocked, and power transmission from the input shaft (2) to the transmission shaft (3) can be blocked. Via a second clutch (20), power is transmitted from the transmission shaft (3) to the input shaft (2), while power transmission from the input shaft (2) to the transmission shaft (3) is blocked. Thus, when driving using only the power of the engine (111), power transmission from the input shaft (2) to the transmission shaft (3) is blocked, thereby preventing transmission of power to a generator/motor (112). Consequently, the generator/motor (112) can be made smaller and lighter, and energy losses can be prevented.