Abstract: A temperature controller is provided for controlling a coolant temperature in a liquid coolant system including a liquid circulation circuit, in which coolant for cooling a machine circulates, and a refrigeration circuit for cooling the coolant. The operating capacity of a compressor of the refrigeration circuit is variably adjusted by an inverter, and when the operating condition of the machine is changed, the frequency value for driving the compressor is varied in accordance with the change. Accordingly, it is possible to immediately correspond to the change of a liquid temperature to be generated in the future. In addition, an appropriate control mode can be selected as a control for varying the frequency in combination with a normal feedback control, such that the liquid temperature may converge on a set value or may be co-changed to a room temperature.