To provide a power transmission system allowing improvement of power transmission efficiency as preventing that driving force of a load becomes lower than required driving force.

An electronic controller 42 selectively executes one of EV (Electric Vehicle) travel control controlling power transmitted between a motor generator 22 and driving wheels 40 in a state that clutches C1, C2 are released and that a brake B1 is engaged, power assistance control controlling power transmitted between the motor generator 22 and the driving wheels 40 in a state that the clutch C2 is released and that the clutch C1 and the brake B1 are engaged, and power distribution control controlling distribution of power transmitted to a transmitter 14 and a planetary gear mechanism 20 by controlling torque of the motor generator 22 in a state that the brake B1 is released and that the clutches C1, C2 are engaged, and performs a changeover to the power distribution control from the power assistance control when the required driving force of a vehicle is not more than a value capable of being covered by torque of an engine 10.

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