PURPOSE: To attain to simplify circuit constitution, by informing the abnormality of display by the reversal of a display signal for a definite time after the start of measurement.

CONSTITUTION: An input signal is converted by a AD converter 1 to be stored in memory 2 and the content of the memory 2 is read to be supplied to a display signal generation circuit 3 where a display signal is formed. This display signal is applied to a display part 5 through a display signal reversal circuit 4 and a wave form is displayed on a display device5 by scanning. The output of the circuit 3 is usually applied to the display part 5 without performing level reversal by the circuit 4. A frequency comparing circuit 8 cmpares the output frequency of a wave form shaping circuit 6 for triggering and the frequency of the time base clock of a time base oscillation circuit 7 to discriminate whether a signal is normal on the basis of a set value and the output of the circuit 8 is added t the circuit 4 as a reversal order and, when the level thereof is a H-level, the output of the circuit 3, that is, the level of a display signal is reversed. By this method, circuit constitution is simplied and cost can be reduced.