PURPOSE: To provide a small-sized PIN amplifier having the conventional functions by moving a specific conductor pattern from a first substrate to a second substrate.

CONSTITUTION: A first-layer substrate X has a pattern of conductor islands I1-I8, on which are connected a photodiode PD, an integrated circuit chip IC, and capacitors C1 and C2. A second-layer substrate Y includes rectilinear conductors L1-L7, and a third-layer substrate Z includes conductor pads P1-P6. The rectilinear conductor L7 on the substrate Y is the conductor that, in the prior art, is provided outside the region where the elements PD, C1, C2, IC on the first-layer substrate. In this configuration, therefore, the conductor pattern occupies a smaller area on the substrate X.