Abstract: In order to provide a photo detector in which the generation of spontaneous noise is suppressed and external noise is effectively reduced, a p-type diffusion layer is provided on the obverse side of an n-type semiconductor substrate, a p-type low-resistance layer is provided on the obverse side of the p-type diffusion layer, a lead frame is provided on the reverse side of the n-type semiconductor substrate with an insulative resin film in between, and the p-type low-resistance layer is electrically connected to the lead frame. With this structure, the obverse surface of the photo detector is held at the ground potential and the intrusion of electromagnetic noise from outside, that is, external noise can be prevented, so that the reception distance can be increased. In addition, since the junction between the p-type diffusion layer and the p-type low-resistance layer is not a pn junction, the generation of spontaneous noise is sufficiently suppressed.