An object of the invention is to reduce the size of a probe for a photoacoustic measurement device. A light guide 71 is arranged such that one of a two side surfaces 71a is closer to a probe axis C which faces a subject than the other side surface and a light emission end surface 71c is closer to the probe axis C than a light incident end surface 71b when the probe is used. When a refractive index of the light guide 71 with respect to the light is n1 and a refractive index of a medium around the light guide with respect to the light during photoacoustic measurement is n2 (n2
