exact thermocouple output can be detected if an inverting amplifier be adopted for this amplification the condition of r in gt gt r2 might not be satisfied the adder block 17 essentially comprises an inverting amplifier a2 and resistors r7 rb r11 and r13 to the thermister th a resistor r10 is connected in parallel and the source voltage v dd is fed through a resistor r9 resistance values may for example be r7 r8 100 k u03c9 r11 r13 47 k u03c9 r9 220 k u03c9 and r10 50 k u03c9 disposed between the adder block 17 and the microcomputer unit 21 is an open close switch sw which is controlled from the output port port2 fig9 in the adder block 17 the input from the inverting amplifier a1 is applied to the inverting amplifier a2 through resistor r7 and the input from thermister th is fed to the amplifier a2 through resistor r11 moreover the voltage made available by dividing the source voltage v dd by resistor r12 and variable resistor vr1 is fed to the inverting amplifier a2 through resistor r13 since the output of the inverting amplifier a2 is added to the a d converter of the microcomputer unit 21 the output of the inverting amplifier a2 must be constantly maintained in the plus range regardless of the temperature dependent change of the output voltages from the heat sensor complex 2 and thermister th therefore in the present invention the variable resistor vr1 is adjusted so that the output of the inverting amplifier a2 will always be within the range of 0 v 2 55 v as shown in fig9 the temperature setting block 20 comprises resistors r14 r15 a buffer a3.