A power source device includes a main power source for outputting a direct current voltage with a specific value; a monitoring unit connected to the main power source for comparing the direct current voltage with a reference voltage to monitor whether a malfunction occurs in the main power source; and a monitoring display unit connected to the monitoring unit. The monitoring unit includes a comparison unit for comparing the direct current voltage with the reference voltage, and a monitoring control unit for detecting a signal of the comparison unit and for storing and displaying the direct current voltage on the monitoring display unit when the malfunction occurs in the main power source.