To solve the problem where proper flow rate for freezing mixture cooling a superconducting coil, in which conventionally since the ratio between gas phase and liquid phase of freezing mixture flowing in a superconducting coil is unknown, proper (minimum) flow rate for the freezing mixture cooling a superconducting coil is not obtained.

In this monitoring equipment, the pressure above and below the periphery of a sperconducting coil, where a void ratio is to be measured, is measured with a differential pressure type electrical transducer, and the void ratio is obtained from the pressure. When the void ratio exceeds a set value, signals are transmitted to the actuator of a valve to control the flow rate of freezing mixture, opening and closing of the valve is performed.