A surface illuminating light source device comprising: a light source comprising one or a plurality of light radiating bodies; an optically transmissive light guide propagating light from the light source; a casing enclosing the light guide except for an open side defining a radiation surface at a prescribed position in the radiation direction and having an inner reflection means formed of a bottom surface and a side reflection section that reflect light, in which the light source is arranged substantially at a center of the bottom surface; and a radiation side reflection means (20, 30, 40) arranged on the open side, wherein the radiation side reflection means (20, 30, 40) has an outer reflection section (22, 32 1...32 n, 42) with a partially reflective surface that reflects light on the surface facing the light source at a prescribed rate and makes a corresponding amount of light pass therethrough, wherein an opening section (24, 34, 44) comprising multiple through holes (24 1..i, 34 1..i, 44 1..i) is formed on a periphery of the outer reflection section and through which only reflection light reflected at least once on one of the reflection surfaces among the light from the light source passes, characterized in that when the distance along the radiation surface from the center of the radiation surface facing the light source to a given through hole (24 1..i, 34 1..i, 44 1..i) is x, distance from the light source to the radiation side reflection means (20, 30, 40) in the radiation direction is d, an opening dimension of the through hole is a, and the thickness of the radiation side reflection means is t, d / x < t / a is satisfied. a light source comprising one or a plurality of light radiating bodies; an optically transmissive light guide propagating light from the light source; a casing enclosing the light guide except for an open side defining a radiation surface at a prescribed position in the radiation direction and having an inner reflection means formed of a bottom surface and a side reflection section that reflect light, in which the light source is arranged substantially at a center of the bottom surface; and a radiation side reflection means (20, 30, 40) arranged on the open side, wherein the radiation side reflection means (20, 30, 40) has an outer reflection section (22, 32 1...32 n, 42) with a partially reflective surface that reflects light on the surface facing the light source at a prescribed rate and makes a corresponding amount of light pass therethrough, wherein an opening section (24, 34, 44) comprising multiple through holes (24 1..i, 34 1..i, 44 1..i) is formed on a periphery of the outer reflection section and through which only reflection light reflected at least once on one of the reflection surfaces among the light from the light source passes, characterized in that when the distance along the radiation surface from the center of the radiation surface facing the light source to a given through hole (24 1..i, 34 1..i, 44 1..i) is x, distance from the light source to the radiation side reflection means (20, 30, 40) in the radiation direction is d, an opening dimension of the through hole is a, and the thickness of the radiation side reflection means is t, d / x < t / a is satisfied.