voltage value anin is also reduced the a d converter 64 converts under the control of the cpu 3 the variable analog voltage value anin to a digital value in accordance with the magnitude of the analog voltage value anin the resulting digital value is read by the cpu 3 and the cpu 3 in turn gives the voltage specifying signal to the d a converter 25 with the above constitution an automatic contrast controlling function can be realized fig1 is a flow chart showing the procedure performed in the cpu 3 at this time at first the reference value of the a d converter 64 and the reference value of the d a converter 25 in normal conditions are set step s1 then the analog voltage value anin outputted from the temperature detector 66 at a specified period is subjected to the a d conversion step s2 so as to judge whether or not the value resulting from the a d conversion is same as the above reference value set value of the a d converter 64 step s3 when it is judged to be same the set value of the d a converter 25 is reset to the original set value step 4 thereby returning to step s2 when it is judged that the resultant value is not same as the set value in step s3 it is further judged whether or not the resultant value is larger than the set value step s5 when the resultant value is larger than the set value the set value of the d a converter 25 is reduced.