A main body unit (M) is provided with: a voltage detecting unit (11), which generates voltage detection signals with a dropped voltage of a circuit voltage to be measured, and which outputs the generated voltage detection signals to a signal line (12); a correction information storage unit (14a), which stores correction information for correcting errors of the voltage detection signals; and an operation unit (14), which corrects the voltage detection signals using the correction information stored in the correction information storage unit (14a), and which obtains power using the corrected voltage detection signals. An additional unit (Si) is provided with: an information acquiring unit (23a), which acquires the correction information from the main body unit (M); and an operation unit (23), which corrects the voltage detection signals using the correction information acquired by the information acquiring unit (23a), said voltage detection signals having been inputted via a signal line (21), and which obtains power using the corrected voltage detection signals.