A transmission state determination method of a power transmission device for an internal combustion engine driven vehicle including steps of: driving a generator by an internal combustion engine with a vehicle driven by the internal combustion engine stopped; performing an intake air amount gradually increasing process for gradually increasing the intake air amount of the internal combustion engine when the internal combustion engine is controlled so as to generate predetermined power from the generator; determining that the power transmission device for transmitting the output of the internal combustion engine to wheels of the vehicle is in a state of transmitting power when slight movement of a vehicle body is detected in the intake air amount gradually increasing process; and determining that the power transmission device is in a state of cutting power transmission when movement of the vehicle body is not detected in the intake air amount gradually increasing process.