In a monitoring system for detecting a quench in a superconductive coil to which AC power is supplied, a switch apparatus is connected between the coil and a A.C. power source. A voltage between a top of the coil and a dividing resistor connected in parallel with the coil is detected and converted into a voltage signal by a insulation amplifier and a current flowing through the coil 1 is detected by a current detector and is converted into a current signal by an amplifier. The voltage signal is detected with the current signal as a reference signal by a phase detector so that a signal component of the voltage signal in phase with the reference signal is outputted from the phase detector. The signal component is averaged in a averaging circuit, when the voltage signal is out-of-phase with the reference signal, an output signal is generated as an alarm signal from the averaging circuit.