PURPOSE: To reduce electric power necessary for the control of temp. and to control temp. within a wide temp. range.

CONSTITUTION: A heater 10, a thermoelectric cooler 2, a fan 14 and a temp. sensor 20 are provided in a thermostatic chamber 2 and a temp. sensor 22 is provided to the outside of the thermostatic chamber. The cooler 12 is turned ON when the temp. difference T2(=To-Ts) between set temp. Ts and external temp. To exceeds a constant T20 and turned OFF when the temp. difference T2 is less than a constant T21(T20). The rotational speed of the fan 14 is set to a constant high speed K3 when T2 is a constant T23 or less and reduced corresponding to T2 when T2 exceeds T23.