A power transmission system for a four-wheel drive vehicle comprising: a power source, and a power transmission device which includes a gear case, and a transfer case (7A, 7B, 7C, 7D) that is supported by the gear case and that distributes power from the power source to main drive wheels (6L, 6R) and auxiliary drive wheels (10L, 10R), wherein: the power source includes an engine; and the transfer case includes a transfer input shaft that is connected to a differential to the main drive wheels and that is arranged in the vehicle width direction, a transfer output shaft that is connected to the auxiliary drive wheels by a propeller shaft and that is arranged in a longitudinal direction of the vehicle, a power transmission mechanism (75, 76, 78) that transmits power by changing the direction of an axis of rotation to an orthogonal direction between the transfer input shaft and the transfer output shaft and transmits the power, and a rotatable member (75b, 76b, 78b) of the power transmission mechanism, which rotatable member is provided on the transfer output shaft and is disposed in a position overlapping the gear case in the vehicle width direction, characterized in that: the engine is supported on one of two side surfaces of the gear case; the power source further includes an electric motor (2, 3) that is supported and located on the other side surface of the two side surfaces and that is smaller than the engine in the vehicle width direction, the other side surface being opposite to the side surface on which the engine is supported; the transfer case is supported on the same side surface of the two side surfaces of the gear case as the side surface on which the electric motor is supported; and the transfer case is supported by the gear case and wraps around from the side surface on which the electric motor is supported to a rear surface as seen from above in a layout configuration of a front-side power transmission system for providing a space that is surrounded by a vehicle-rear end surface of the engine, a dash panel, and a vehicle body side panel. a power source, and a power transmission device which includes a gear case, and a transfer case (7A, 7B, 7C, 7D) that is supported by the gear case and that distributes power from the power source to main drive wheels (6L, 6R) and auxiliary drive wheels (10L, 10R), a gear case, and a transfer case (7A, 7B, 7C, 7D) that is supported by the gear case and that distributes power from the power source to main drive wheels (6L, 6R) and auxiliary drive wheels (10L, 10R), wherein: the power source includes an engine; and the transfer case includes a transfer input shaft that is connected to a differential to the main drive wheels and that is arranged in the vehicle width direction, a transfer output shaft that is connected to the auxiliary drive wheels by a propeller shaft and that is arranged in a longitudinal direction of the vehicle, a power transmission mechanism (75, 76, 78) that transmits power by changing the direction of an axis of rotation to an orthogonal direction between the transfer input shaft and the transfer output shaft and transmits the power, and a rotatable member (75b, 76b, 78b) of the power transmission mechanism, which rotatable member is provided on the transfer output shaft and is disposed in a position overlapping the gear case in the vehicle width direction, characterized in that: the power source includes an engine; and the transfer case includes a transfer input shaft that is connected to a differential to the main drive wheels and that is arranged in the vehicle width direction, a transfer output shaft that is connected to the auxiliary drive wheels by a propeller shaft and that is arranged in a longitudinal direction of the vehicle, a power transmission mechanism (75, 76, 78) that transmits power by changing the direction of an axis of rotation to an orthogonal direction between the transfer input shaft and the transfer output shaft and transmits the power, and a rotatable member (75b, 76b, 78b) of the power transmission mechanism, which rotatable member is provided on the transfer output shaft and is disposed in a position overlapping the gear case in the vehicle width direction, characterized in that: the engine is supported on one of two side surfaces of the gear case; the power source further includes an electric motor (2, 3) that is supported and located on the other side surface of the two side surfaces and that is smaller than the engine in the vehicle width direction, the other side surface being opposite to the side surface on which the engine is supported; the transfer case is supported on the same side surface of the two side surfaces of the gear case as the side surface on which the electric motor is supported; and the transfer case is supported by the gear case and wraps around from the side surface on which the electric motor is supported to a rear surface as seen from above in a layout configuration of a front-side power transmission system for providing a space that is surrounded by a vehicle-rear end surface of the engine, a dash panel, and a vehicle body side panel.