A transmission mechanism including a switching clutch or brake is switchable between a continuously-variable shifting state and a step-variable shifting state, and provides improved fuel economy by a transmission with an electrically variable speed ratio and high power transmitting efficiency by a gear type power transmitting device constructed for mechanical transmission of power. During four-wheel drive vehicle running with second drive wheels driven by a third electric motor, a switching control switches a differential portion to a continuously-variable shifting state, so that an operating speed of a first electric motor operated by an output of an engine as an electric generator is controlled to assure a higher degree of electricity generating efficiency, owing to the differential function of the differential portion, than when the differential portion is placed in a non-continuously-variable shifting state in which first electric motor speed and engine speed are determined by vehicle running speed.