PURPOSE: To provide a mounting/connecting substrate of the multipin circuit element which contributes to miniaturization of devices by decreasing the substrate area and is realized at a low cost and narrow pitch.

CONSTITUTION: Land parts 13 for pin soldering of a photodetector 17 are formed by solder plating layers on the mounting/ connecting substrate of the photodetector which is formed by successively laminating a metallic layer 11, an insulating layer 12 and the solder plating layer, is mounted with the photodetector 17 on a device to be mounted and connects the photodetector 17 and flexible printed circuit boards 15. These land parts 13 are provided with a through-hole 14 for inserting the flexible printed circuit boards 15 and connecting these boards to the land parts 13 near the land parts 13.