To measure with ease an input voltage from an external power source and notify the user of the input voltage, a power source apparatus comprising a power source voltage input terminal, an insulating transformer, a power source control device, and a communication unit is provided. The power source voltage input terminal is connected to an external battery. The insulating transformer has a primary coil that is connected to the power source voltage input terminal. The power source control device is connected to the power source voltage input terminal and measures the input voltage to the power source voltage input terminal. The communication unit transmits the voltage value measured by a microcomputer to the secondary side of the insulating transformer.