A motor-generator is feedback-controlled by adopting the rotation speed information from a resolver when the rotation speed of a motor-generator is low, and by adopting the rotation speed information from a north marker when the rotation speed thereof is high. If the gradient of change in the rotation speed thereof based on the presently adopted rotation speed information is greater than or equal to a predetermined value, the rotation speed information switching operation is prohibited to prevent the shift shock during the ratio shifting operation of a reduction mechanism. That is, in a construction in which the rotation speed of the motor-generator can be detected via the resolver and the north marker, the power output device capable of properly performing the rotation speed control of the motor-generator and the like by making proper the timing at which the rotation speed information adopted for the control is switched.