To lower equipment costs and a running cost by enabling supply of cooling water in the specified order.

Supply pipes 1 and 6 and return pipes 2 and 7 are provided to circulate cooling water separately to two refrigerating machines AR and TR connected in series and condensers of both the refrigerating machines and a temperature control valve 4 is provided to control the temperature of the cooling water by guiding the cooling water of the return pipe 2 of one refrigerating machine to the supply pipe 1. Two bypass pipes 11 and 12 are provided to be connected between the return pipe 2 of one refrigerating machine and the supply pipe 6 and the return pipe 7 of the other refrigerating machine thereby enabling supply of the cooling water to the one refrigerating machine and the other refrigerating machine sequentially.