PURPOSE: To display a continuous variation of the quantity of input so as to be more approximate, by adding a variable value derived from a differnce between the previous counting data and the present data which has been counted newly, to a display output data whenever the display output is switched, and obtaining the display output which is changed stepwise.

CONSTITUTION: A counting circuit 3 counts the quantity of input at every constant interval of time, obtains a counting data, and derives a difference between the present counting value and the pevious counting value by a subtracting circuit 5. An output of this circuit 5 is divided 6 by the number of times for switching the display within an optionally set constant interval of time, a variable value outputted from the circuit 6 is sent to an adding circuit 7, and is added to a displaying output. In this way, it is possible to obtain a display output which is updated in order and stepwise whenever the display is switched.