PURPOSE: To enhance control accuracy and to simplify machinery constitution by directly detecting the temp. of a reaction tank to control the same by a microcomputer.

CONSTITUTION: The water heated by a heater 12 and a heater 13 having constitution wherein water flows around the heater 12 is allowed to flow in a reaction tank 15 by a pump 24 through a pipe 14 and this thermostatic water 16 controlled in its temp. is recirculated to the reaction tank 15 to make the temp. of the reaction cell 17 mounted to a reaction table 18 constant. The temp. of the heater 13 is detected by a temp. sensor 1 to be inputted to a control CPU through a converter 3, a multiplexer 4 and an A/D converter 5 and kept constant by controlling the heater 12 through a pulse width modulator 9 and an SSR 10. The temp. of the reaction tank 15 is detected by the temp. sensor 2 and inputted to the control CPU 6 in the same way as the temp. sensor 1. Then, the set value of the temp. control of the heater 13 is determined so as to make the temp. of the reaction tank 15 constant.