An aspect of the invention relates to an electric power conversion system. The electric power conversion system includes: a first battery; a second battery; an electric power converter including a plurality of switching elements, the electric power converter being configured to bidirectionally step up or step down electric power between an output line and each the first and second batteries by turning on or off the plurality of switching elements in accordance with PWM signals; and a controller configured to control a first step-up and step-down circuit and a second step-up and step-down circuit by generating a first PWM signal and a second PWM signal, the first step-up and step-down circuit being established between the first battery and the output line, the first PWM signal being a signal for controlling step-up and step-down operation of the first step-up and step-down circuit, the second step-up and step-down circuit being established between the second battery and the output line, the second PWM signal being a signal for controlling step-up and step-down operation of the second step-up and step-down circuit, the controller being configured to, at the time of coupling on-duty periods of both the first and second PWM signals with each other by shifting a phase of at least one of the first and second PWM signals, execute an overlap phase shift that partially overlaps the on-duty periods of the first and second PWM signals by raising one of the PWM signals before the other one of the PWM signals falls.