To acknowledge a location requiring maintenance work and its maintenance time before a disturbed state occurs and cause a down-time in operation of a constant- temp. liquid circulating device to be shortest in length by a method wherein each of the segments in the device is provided with a sensor for use in monitoring various kinds of state amounts in a continuous manner.

Each of the segments in the device is provided with a sensor for use in monitoring various kinds of state amount in a continuous manner. As these kinds of sensors, it is possible to mount at requisite locations in a constant-temp. liquid circulating device, a temperature sensor for use in sensing temperatures at each of a cooling circuit and a circulating circuit, a current sensor for sensing a current value of each of a motor for driving a compressor 11 or a pump 24 or other electrical driving segments, a pressure sensor for use in sensing a pressure of refrigerant or refrigerant liquid at each of the segments in a refrigerant flow passage at a cooling circuit and a circulating circuit, a flow rate sensor for use in sensing a flow rate of the refrigerant liquid in the refrigerant flow passage in the circulating circuit, or various kinds of sensors for use in detecting variation in state amount effective for estimating other irregular operations in the constant-temp. liquid circulating device or the like.