circuit 5 outputs a difference signal v d in proportion to the difference between the reference voltage v f and the output from the amplifier circuit 4 the difference signal v d is then digitalized by an a d converter 6 and then inputted to a microcomputer 7 which comprises a cpu 7a a ram 7b and a rom 7c the microcomputer 7 generates operation signals to operate a thermomodule driving circuit 8 in accordance with the difference signal v d supplied with a current from the thermomodule driving circuit 8 the thermomodule unit 2 supplies heat to or absorbs heat from the flow cell 1 to control the same to be at a predetermined aiming temperature the detailed function of the embodiment having such a constitution is described in the following on reference also to the formulae and fig4 mentioned in the preceding section of objects and summary of the invention suppose for instance that a relation t e u2266 t lt lt t a exists in the beginning where t e is the environmental temperature around the flow cell 1 t the temperature of the flow cell 1 and t a an aiming temperature the inverting amplifier circuit 4 with the terminal 4b kept at a minus potential v 0 outputs a temperature signal of a voltage v 0 r r t according to the resistance r t of the thermistor 3 which resistance r reflects the temperature t the differential amplifier circuit 5 compares the temperature signal with a positive reference voltage v f which is set at a value corresponding to an aiming temperature t a at which the flow cell 1 is to be finally maintained therefore the reference voltage v f is equal to v 0 r r t a comparing v 0 r.