A photodetection element is a photodetection element having an incidence surface for light on a back surface of a semiconductor layer, and includes a periodic nano-concave/convex structure provided on a front surface of the semiconductor layer and having convex portions and concave portions constituting a longitudinal resonator and a transverse resonator for the light incident from the incidence surface, the periodic nano-concave/convex structure converting the light into surface plasmons, and a metal film provided to cover the periodic nano-concave/convex structure, a height and an arrangement pitch of the convex portions in the periodic nano-concave/convex structure are set such that a resonance wavelength of the longitudinal resonator and a resonance wavelength of the transverse resonator match, and a thickness of the metal film is equal to or greater than 20 nm.