To enable simultaneous observation both in a bright region and a dark region by multiplying a first coefficient when a digital signal converted from an analogue signal output from a secondary charged particle detector is larger than a reference value, and multiplying a second coefficient when it is not larger.

An output signal from an ADC is input to a comparator 78 and it is compared with a reference value. When it is smaller than the reference value, a coefficient is multiplied by a multiplying means, and when larger than that it is subjected to signal processing by a signal delaying means for delaying the signal by a time the same as that required therefor passing through the multiplying means. The ADC, comparator 78, multiplying means, and delaying means perform signal processing in synchronization with the same clock signal. A latch function is given to each part, whereby a pipe line processing is made possible to implement the signal processing without a delay. The signal is input to a DSP and calculation processing and another signal processing are implemented in real time. A shift circuit is used for the multiplying means, whereby a simple configuration is attained.