A power output system that provides power to a driveshaft is provided which includes an internal combustion engine, an electric motoring mechanism, an electric storage device, a first exhaust gas channel that guides exhaust gas from the engine directly to an exhaust gas purifying catalyst, a second exhaust gas channel which guides the exhaust gas from the engine to the exhaust gas purifying catalyst via an unburned-component adsorber, a channel switching mechanism operable to switch a channel of the exhaust gas from the engine between the first exhaust gas channel and the second exhaust gas channel, a pressure accumulator operable to accumulate therein a negative pressure produced through rotation of the engine, a negative-pressure introducing/releasing mechanism that selectively permits and inhibits introduction of the negative pressure from the pressure accumulator into the channel switching mechanism, and a controller that controls the electric motoring mechanism and the negative-pressure introducing/releasing mechanism.