A light irradiation apparatus for irradiating a microplate having a main surface on which a plurality of wells is two-dimensionally arranged to accommodate an object to be measured and a back surface opposing the main surface, with measurement light from the back surface side of the microplate, the apparatus comprising: a light source configured to output measurement light to be irradiated to each of the plurality of wells from the back surface side of the microplate; and a light guiding member which is arranged so that the measurement light outputted from the light source propagates through it, the light guiding member having a main surface that is arranged to directly face the back surface of the microplate, a back surface opposing the main surface, and a side surface which intersects the main and back surfaces and which is arranged so that the measurement light from the light source is incident on it, wherein the light guiding member has a plurality of light-emitting units arranged on the main surface thereof, each corresponding to a respective one of the plurality of wells of the microplate, and each configured to control the direction of emission of the measurement light so as to prevent the measurement light having propagated through the light guiding member from being incident to the back surface of the microplate from a perpendicular direction thereof, wherein each of the plurality of light emitting units includes one or more depressions (61e, 61g) each extending from the main surface toward the back surface of the light guiding member and having an opening on the main surface, and wherein the measurement light, inputted into the light guiding member from the side surface thereof, is refracted and reflected at the side surface of each of the depressions, and then outputted from the opening of each of the depressions. a light source configured to output measurement light to be irradiated to each of the plurality of wells from the back surface side of the microplate; and a light guiding member which is arranged so that the measurement light outputted from the light source propagates through it, the light guiding member having a main surface that is arranged to directly face the back surface of the microplate, a back surface opposing the main surface, and a side surface which intersects the main and back surfaces and which is arranged so that the measurement light from the light source is incident on it, wherein the light guiding member has a plurality of light-emitting units arranged on the main surface thereof, each corresponding to a respective one of the plurality of wells of the microplate, and each configured to control the direction of emission of the measurement light so as to prevent the measurement light having propagated through the light guiding member from being incident to the back surface of the microplate from a perpendicular direction thereof, wherein each of the plurality of light emitting units includes one or more depressions (61e, 61g) each extending from the main surface toward the back surface of the light guiding member and having an opening on the main surface, and wherein the measurement light, inputted into the light guiding member from the side surface thereof, is refracted and reflected at the side surface of each of the depressions, and then outputted from the opening of each of the depressions.