PURPOSE: To control the temp. ripple within the measuring error range by passing a stabilized boiling heating medium through a heat exchanger from the outside.

CONSTITUTION: With respect to the thermostatic bath for keeping the temp. of the liq. in the thermostatic bath at a fixed value used for the measurement of the temp. characteristic such as the inspection of the temp. instrument, etc., the heat exchanger 10 is furnished in the liq.-filled part 3a of the thermostatic bath 1, an external heater 11 is provided separately from a vessel 3, the external heater 11 and the heat exchanger are connected with a circulating pipeline 12, and a heating medium which has been heated by the external heater 11 upto its b.p. is circulated to the heat exchanger 10 in the thermostatic bath through the pipeline 12. The stabilized boiling medium is passed through the heat exchanger from the outside in this way, the temp. ripple is controlled within the measuring error range, and hence measurement can be carried out with high precision even where the temps. of many analytes in the specified range must be measured in a short time.