A computing section controls a hot swap circuit, and DC/DC converters based on binary data from an ADC and an interrupt signal from respective comparing sections of a voltage detecting circuit. If the interrupt signal is input from any one of the respective comparing sections, the reading of binary data corresponding to the interrupt signal is halted. The binary data relating to the voltage detecting section corresponding to the interrupt signal, which is held in the SRAM at that time, is transferred to a non-volatile memory, where it is stored. When driving of the system has been halted, binary data is read out from the non-volatile memory, in order to analyze the cause of the error in a DC/DC converter which indicates an error output voltage from one of power supply circuits for supplying externally a plurality of different drive voltages to loads in a disk array unit.