PURPOSE: To realize a high sensitivity circuit for detection by providing a photodetector, a transmission line connected to the photodetector in parallel and an amplifier element combined to the photodetector, and permitting the transmission line to resonate in parallel with an electrostatic capacity components that the photodetector and the amplifier element have and with plural frequencies.

CONSTITUTION: At frequencies (f1, f2 and f3) in which the absolute value of an inductive reactance component in the transmission line 52 and that of a capacitive reactance component become equal, a parallel resonance occurs between the transmission line 52 and the electrostatic capacity C1. At these plural resonance frequencies, an impedance Zin which is viewed from the input level stage (terminals 53-53') of an amplifier A in an equiavalent circuit becomes a resistance equivalent to the negative resistance RL of the photodetector, and the impedance Zin at the time of resonance becomes the maximum value RL, compared to the impedance Zin at other frequencies. Consequently, at the resonance frequencies (f1, f2 and f3) a current arising in the photodetector is effectively converted into a voltage only by the negative resistance RL, therefore, noise in the photodetector can be reduced.