Abstract: A control system for effecting ultra-precision temperature control for a controlled system having large load fluctuations or including a dead time component. A control unit (5) instructs a high-precision PID temperature controller (10) to renew and correct a control target value T.sub.0,NEW for the supply fluid temperature T.sub.0 only by a temperature difference .DELTA.T=T.sub.C -T.sub.2 between a set value T.sub.C for the return fluid temperature T.sub.2 of a heat transfer fluid and the actually controlled return fluid temperature T.sub.2. In response to the instruction, the high-precision PID temperature controller (10) controls the heating power of a heater (17) for heating control through a constant-power thyristor phase controller (16). The heater (17) heats the heat transfer fluid and sends it to a machining apparatus (1) through a closed damping tank (21). The heat transfer fluid exchanges heat with the machining apparatus (1) and is then returned to a precooling means.