A photodetector module that can achieve impedance matching and power saving. A photodetector (11) and an amplifier (12) for amplifying an electric signal from the photodetector (11) are mounted on a stem (14). A dielectric plate (18) is arranged between the stem (14) and a flexible substrate (20). To transfer an electric signal from the amplifier (12) to the substrate (20), a lead pin (15d) is provided to pass through the stem (14) and the dielectric plate (18). The output of the amplifier (12) includes a capacitance component, and the output impedance of the amplifier (12) is higher than the impedance that matches with the substrate (20). Further, the thickness d of the dielectric plate (18) is such that the inductance component of the lead pin (15d) includes an inductance component that is inductive, which cancels the capacitance component of the amplifier, and impedance matching with the substrate (20) can be achieved.