The present invention configures a power device for a portable working machine by including an engine (20), and an electric motor (50) connected to the engine (20) in a manner such that output torque can be transmitted to the crank shaft (22) of the engine (20). The output torque of the electric motor (50) is changed within the combustion cycle (1) of the engine, and the electric motor (50) biases the crank shaft in the normal rotational direction during a process in which the piston (32) of the engine (20) rises toward the compression top dead center. As a consequence, the present invention provides a power device for a portable working machine, wherein it is possible to stabilize the rotation of the engine without depending on a flywheel.
