PURPOSE: To recover energy of coil rapidly when quenching phenomenon takes place in coil by making protecting device operate through optical fiber and light receiving element in combination with luminous diode housed together with superconductive coil in low-temperature container.

CONSTITUTION: Superconductive coil 2 is housed in low-temperature container 1 kept cool by liquid helium and pumping of coil flux is made by connecting terminals with outer power source 3. Next in a row with coil 2 is connected helipot resistance 4 and between this and middle-point tap of coil 2 is provided a bridge circuit composed of reversely-connected luminous diodes 9a and 9b linked in series with resistance 8. And then optical fiber 10 is arranged in opposition to one end of luminous parts of these diodes 9a and 9b. The other end of optical fiber 10 is led outside of container 1 and faced with phototransistor 11 connected with protecting device. When the quench of coil 2 is detected by diodes 9a and 9b thus constituted, power source 3 is braced up and energy of coil 2 is recovered rapidly.