The purpose of this invention is to provide a flat-layout light-receiving package whereby optical-signal monitoring-reception sensitivity can be made constant without increasing the size of a module. This flat-layout light-receiving package is provided with the following: a first subassembly (31) in which an optical receiver (41) is affixed to the front of a retaining holder (42), the rear surface of which is flat and capable of parallel motion in a plane parallel to the light-reception surface of the optical receiver (41); and a second subassembly (32) that has a flat base (44) laid out on a substrate (11) and a U-shaped concave holder (43) that holds the retaining holder (42), wherein the surface of the flat base (44) that is mounted to the substrate (11) is flat and capable of motion parallel to the surface of the substrate, the concave holder (43) and the flat base (44) are affixed to each other, and at least one of the U-shaped surfaces of the concave holder (43) is flat and allows parallel motion of the rear surface of the retaining holder (42).