A power transmission device includes: an engine; a rotary machine; and an oil pump connected to each of a rotary shaft of the engine and a rotary shaft of the rotary machine through a one-way clutch. The power transmission device is configured to allow the rotary machine to rotate at a rotational speed higher than the rotational speed corresponding to a speed of the engine and lower than the rotational speed corresponding to an idling speed of the engine at the time of startup of the engine.