In a constant temperature liquid circulating device that connects a piping of circulating liquid in an external device between a discharge port and a return port of a conduit for delivering the circulating liquid at a constant temperature by a pump, a flow amount sensor and temperature sensors for detecting discharge temperature (T.sub.1) of the circulating liquid from the conduit and the return temperature (T.sub.2) of the circulating liquid are provided in the conduit, so that the flow amount and the temperature of the circulating liquid is controlled by a controller according to the heat load of the external device obtained on the basis of the outputs from the temperature sensors.