A power transmission system having an input member (6) and an output member (2) for transmitting power, and an oil pump (7) for discharging oil by a relative rotation between a first (8, 70) and a second rotary members (9, 87, 88) which is driven by the power transmitted between the input member (6) and the output member (2), characterized in that: the input member (6) and the first rotary member (8, 70) are connected with each other in a power transmittable manner, and the output member (2) and the second rotary member (9, 87, 88) are connected with each other in a power transmittable manner; and characterized by comprising: a transmission member (11, 13, 76, 77) for connecting the first and the second rotary members in a power transmittable manner; and a control valve (9, 87, 88) for controlling a power transmission state between the first and the second rotary members, by controlling a discharge condition of the oil pump (7).