To obtain a signal component which eliminates the variation of circuit elements included in a signal and a DC fluctuation component that is caused by secular change, etc., by filtering a step-wise differential voltage amplification output that is in proportion to the inclination of an input signal waveform with a filter circuit and eliminating a DC fluctuation component from a signal output.

An amplifier 1 amplifies an analog input signal, and a lowpass filter circuit 2 reduces noise after amplification and inputs it to 1st and 2nd sample hold circuits 3 and 7. An input signal waveform is sampled and held in 1st and 2nd sampling pulse strings, and hold voltage is respectively outputted to 3rd and 4th sample hold circuits 4 and 8. Next, the hold voltage is further sampled and held in a 3rd sampling pulse string and timing is made uniform. Then, the hold voltage from the circuits 4 and 8 is added to a differential amplifier 5, undergone differential voltage amplification, outputted as step-wise output voltage and finally filtered by a lowpass filter circuit 6.