A detector for detecting a state of an electrical storage device includes capacitor blocks formed of a capacitor-voltage equalizing circuit and a capacitor-voltage detecting circuit both coupled to respective capacitors, and a capacitor-block state outputting circuit outputting states of the capacitors by using the signals supplied from the capacitor-voltage detecting circuits. The signals from the capacitor-voltage detecting circuits are transmitted via an optical switch, which works as an electrical indirect coupler, to the capacitor-block state outputting circuit. This structure allows isolating high-voltage oriented wirings completely from signal oriented wirings, so that high reliability can be ensured.