PURPOSE: To successively measure the potential between the max. value-min. value in the arbitrary measuring section on the surface of an oscilloscope tube by comparing operation, by providing a D/A converter, an analogue comparator, an NAND gate, FF and a control operation circuit.

CONSTITUTION: A signal to be measured is supplied to an analogue comparator 2 through an amplifier 1 and compared with the analogue signal subjected to D/A conversion by a D/A converter 6. High potential output is generated on the positive side of the comparator 2 when a voltage value to be measured is larger than the analogue voltage value from the converter 6 and high potential output is generated on the negative side thereof when the voltage value to be measured is smaller than the analogue voltage value from the converter 6. FF 41 is set by the output of an NAND gate 31 and FF 42 is set by the output of an NAND gate 32. The Q-output of FF 41 is supplied to a circuit 5 as a positive peak signal and the Q-output of FF 42 is supplied to the circuit 5 as a negative peak signal. Then, by successive comparison, the max. and min. values of voltage to be measured are converted to digital data to be stored in max. value and min. value registers. Further, the content difference between the max. value and min. value registers is operated to become the potential difference between the max. and min. values of the voltage to be measured.