A circuit disconnection element that can open or short-circuit between a power storage element and a step-up/step-down bidirectional chopper circuit is provided in a power storage system, thereby preventing a self-discharge state in a process of charging. Furthermore, at the time of occurrence of an abnormality, the power storage element is detached from other devices including the step-up/step-down bidirectional chopper circuit. Accordingly, an energy loss of the power storage element at the time of power regeneration can be reduced to improve the energy use efficiency. Further, breakage influences such that the power storage element causes breakage in other devices at the time of occurrence of the abnormality can be prevented, thereby enabling to obtain an AC motor drive device having mounted therein a highly safe power storage system.