To provide a control method for a power transmission device which can prevent the generation of impacts and noises when a speed change mechanism is changed over from a neutral state to a first speed gear without using a brake mechanism. In a control method of a power transmission device for controlling a power transmission device for transmitting a rotational power of a crankshaft of an internal combustion engine to an output side by way of a clutch mechanism and a speed change mechanism, the speed change mechanism is automatically changed over from a neutral state to a first speed gear during a period from a point in time Ts at which an ignition switch is turned on to a point in time at which an engine rotational speed arrives at a steady-state idling rotational speed Nid due to the starting of an internal combustion engine.