To provide a method for injecting liquid helium into a low temperature holder wherein a superconducting coil is incorporated and held and a cooling system of a superconducting generator wherein a helium gas produced in a process of transfer is subjected to gas-liquid separation and only the liquid helium wherefrom the helium gas having no utility value is removed is injected into the low temperature holder, on the occasion when the liquid helium is transferred for injection from a liquid helium storage tank to the low temperature holder through a transfer tube.

The cooling system of the superconducting generator is constituted of a helium refrigerator 24, a device 25 for gas-liquid separation and injection, and the superconducting generator 10.