An electric power assist system usable for an electric assist vehicle, the electric power assist system comprising: an electric motor generating assist power to assist human power of a rider of the electric assist vehicle; a control device (70, 170) generating a signal controlling an electric current to flow in the electric motor to control a magnitude of the assist power to be generated by the electric motor; and an acceleration sensor outputting an acceleration signal representing at least one of an acceleration Gx in a first direction as a travel direction of the electric assist vehicle and an acceleration Gz in a second direction perpendicular to both of the first direction and a road surface, wherein the control device (70, 170) detects an inclination angle of the road surface based on the acceleration signal, and causes the electric motor to generate assist power of a magnitude in accordance with the inclination angle, characterized in that the control device (70, 170) receives a detection signal based on which it is detected whether the electric assist vehicle is at a stop or is running, the detection signal being received from at least one sensor (35, 38, 41, 42, 45, 46, 51, 53) outputting the detection signal, and wherein the control device (70, 170) detects, based on the detection signal, that the electric assist vehicle is in a stop state, detects an inclination angle of the road surface based on the acceleration signal that is output in the stop state, and causes the electric motor to generate assist power of a magnitude in accordance with the inclination angle when the rider starts pedaling the electric assist vehicle such that the assist power at stop state is applied when the electric assist bicycle starts to run up a slope from the state of being at a stop on the slope and decreased when the electric assist bicycle starts, to run down a slope. an electric motor generating assist power to assist human power of a rider of the electric assist vehicle; a control device (70, 170) generating a signal controlling an electric current to flow in the electric motor to control a magnitude of the assist power to be generated by the electric motor; and an acceleration sensor outputting an acceleration signal representing at least one of an acceleration Gx in a first direction as a travel direction of the electric assist vehicle and an acceleration Gz in a second direction perpendicular to both of the first direction and a road surface, wherein the control device (70, 170) detects an inclination angle of the road surface based on the acceleration signal, and causes the electric motor to generate assist power of a magnitude in accordance with the inclination angle, characterized in that the control device (70, 170) receives a detection signal based on which it is detected whether the electric assist vehicle is at a stop or is running, the detection signal being received from at least one sensor (35, 38, 41, 42, 45, 46, 51, 53) outputting the detection signal, and wherein the control device (70, 170) detects, based on the detection signal, that the electric assist vehicle is in a stop state, detects an inclination angle of the road surface based on the acceleration signal that is output in the stop state, and causes the electric motor to generate assist power of a magnitude in accordance with the inclination angle when the rider starts pedaling the electric assist vehicle such that the assist power at stop state is applied when the electric assist bicycle starts to run up a slope from the state of being at a stop on the slope and decreased when the electric assist bicycle starts, to run down a slope.