PURPOSE: To enable a refrigerant constant temperature control device which continuously feeds refrigerant such as liquid nitrogen of low temperature to continuously feed refrigerant of constant temperature through a piping.

CONSTITUTION: When an object to be cooled 2 is cooled down to a constant temperature with refrigerant 1, an upper limit temperature sensor 4 and a lower limit temperature sensor 5 which are previously set at a certain value and an automatic valve 6 are successively provided to a cold-insulating piping 3 in this sequence starting from the refrigerant feed source. When the temperature of the refrigerant 1 reaches the upper limit of temperature, the automatic valve 6 is opened by signals outputted from the upper limit temperature sensor 4 to start the supply of refrigerant 1 to be the object to be cooled 2. When the refrigerant 1 is cooled to the lower limit of temperature, the automatic valve 6 is closed by signals outputted from the lower limit temperature sensor 5 to stop the supply of refrigerant 1 to the object to be cooled 2.