To provide a weak signal extracting circuit, which has simple construction and can extract a weak signal.

A weak signal extracting circuit 10 comprises a band elimination filter(BEF) 30, an analog subtracter 31, an amplifier 32, and a voltage comparator 33. The band elimination filter 30 eliminates only frequency components of a weak signal, and passes other weak frequency components. An input signal to the weak signal extracting circuit 10 and a signal left after eliminating the weak signal from the input signal with the input signal got through the band elimination filter 30 are input to the analog subtracter 31. The analog subtracter 31 outputs a differential voltage between the two signals, and consequently extracts the weak signal only.