Cooling method of superconducting cable line

To provide a cooling method of a superconducting cable line wherein moistures contained in the cable in the superconducting cable line after laid is removed before cooling, so that solidification of the moistures can be prevented from clogging of piping including a coolant flow channel and a return pipe. There is provided an inert gas generating vessel so that before the coolant is flown into the superconducting cable line , the inert gas is blown into the cable line from an inflow pipe connected with a supply pipe ′ and is discharged from a discharge pipe together with the gas. The inert gas is preheated by a heater before feeding it into the cable line. The discharge pipe connected with a discharge side B of the return pipe is provided with a moisture measuring instrument to measure a moisture content of the discharged gas.