An optical detector includes a semiconductor base portion of a first conductivity type having a first surface and a second surface and provided with a projection projecting from the second surface, a first metal electrode layer provided on the first surface or the second surface of the semiconductor base portion, a semiconductor layer of a second conductivity type having a first portion covering the second surface of the semiconductor base portion and second portions covering side surfaces of the projection, and a second metal electrode layer provided in close contact with the second semiconductor layer such that the projection and the second portions of the second semiconductor layer are interposed therebetween. An MIM resonator is constituted by the projection, the second portions of the second semiconductor layer, and the second metal electrode layer between which the projection and the second portions of the semiconductor layer are interposed.