The system makes it possible to monitor various electrical variables by using a simple structure at low cost with ease and with high reliability by a circuit breaker which jointly includes a current transformer to detect current, a transformer to detect voltage, and a zero-phase current transformer to detect leakage current, in addition to the switch controller to perform the basic function of the circuit breaker. Moreover, the system also provides a function to display various measured values, obtained as described above, by a display unit that can be detached and installed separately from the measuring section to display data sent in digital form by data transmission.