PURPOSE: To permit to carry out easily from the pre-cooling of a cryocooling section to a stationary operation by only one set of supplying valve without using a pre-cooling valve and a stationary valve and permit to supply the freezing medium containing little constituents of gas phase stably to the cooling section upon the stationary operation with a constant speed.

CONSTITUTION: The freezing medium, sent out of a refrigerating machine or a reservoir 5, is supplied to a gas-liquid separator 1 via a supplying pipeline 3 and the supplying valve 4 upon the stationary operation as same as the case of pre- cooling operation and the gas and liquid phases are separated in the freezing medium while the freezing medium, containing little contents of gas phase, is supplied into said cooling section 2 from the lower part of the cryo-cooling section 2 through a natural circulation pipeline 11. The evaporated gas, generated by receiving a heat load in the cryo-cooling section 2, generates a buoyancy while the buoyancy becomes the circulation driving power of a circulating line connecting the gas-liquid separator 1, the natural circulation pipeline 11, the cryo-cooling section 2 and a gas extracting pipeline 6 and the gas is supplied efficiently to the cryo-cooling section 2, therefore, the temperature of the cryo- cooling section 2 may be kept in stable.