To seal a mixed liquid of refrigerants with different boiling points in a high temperature side secondary refrigerant circulation circuit dissipating heat generated by a Sterling refrigerating machine, and to automatically change a mixing ratio of the refrigerants at operation and at stoppage.

The Sterling refrigerating machine 1 is provided with a warm head 2 and a cold head 3, an interior of the cooling storage is cooled by cold of the cold head 3. Heat of the warm head 2 is dissipated by a first high temperature side secondary refrigerant circulation circuit 10 and a second high temperature side secondary refrigerant circulation circuit 30. A mixed liquid of ethyl alcohol and water is sealed as a secondary refrigerant in the high temperature side refrigerant circulation circuits. An isolation tank 40 taking in vaporized/condensed ethyl alcohol before water and isolating it from a secondary refrigerant circulation flow is arranged in the first high temperature side secondary refrigerant circulation circuit 10. When operation of the Sterling refrigerating machine 1 stops, an electromagnetic control valve 51 in a return circuit 50 connected to the isolation tank 40 is opened by a control part 60, and the ethyl alcohol is returned to the first high temperature side secondary refrigerant circulation circuit 10.