In a power transmission system for transmitting power of an engine to a load, a planetary gear mechanism provided in parallel to a speed variator combines torque conveyed from the engine with torque conveyed from a motor generator in a state such that their torque ratio equals a predetermined ratio, and transmits the combined torque to a driven wheel. When performing power transmission between the engine and the driven wheel via both the speed variator and the planetary gear mechanism, power distribution between the power conveyed to the speed variator and the power conveyed to the planetary gear mechanism can be actively controlled by adjusting the torque of the motor generator. As a result, the speed variator capacity can be reduced while enhancing power transmission efficiency.