When an electricity storage unit is being charged or discharged, a control circuit obtains efficiency Efn of a DC/DC converter based on input/output voltage value Vi, input/output current value I and electricity storage unit voltage value Vc detected by an input/output voltage detection circuit, an input/output current detection circuit, and an electricity storage unit voltage detection circuit, respectively. By using temperature T a detected by temperature sensor, when efficiency Efn lies out of predetermined range (from Emin to Emax) according to input/output voltage value Vi, input/output current value I, electricity storage unit voltage value Vc and temperature T, at least any one of an electricity storage unit and the DC/DC converter is determined to be abnormal.