An optical sample detection system that detects a sample by irradiating a sensor chip with excitation light from a light source unit, the sensor chip comprising: a dielectric member; a metal film adjacent to an upper surface of the dielectric member; and a sample solution holding member arranged on an upper surface of the metal film, wherein the sensor chip is an excitation light reflector, and wherein the optical sample detection system emits excitation light from the light source unit and further determines a position of the sensor chip using reflected light of the excitation light reflected at the excitation light reflector, wherein the optical sample detection system further comprises an excitation light detection unit that detects, out of the reflected light, reflected light traveling in a predetermined direction and a fluorescent detection unit, wherein the light source unit is part of a light irradiation unit, wherein a conveyance stage is actuated to move the sensor chip to a measurement position, wherein the measurement position represents a position where the excitation light irradiation unit irradiates the sensor chip with the excitation light and the fluorescence detection unit detects the fluorescence generated with the irradiation, characterised in that: a diffracted light removal slit is provided, wherein the diffracted light removal slit comprises: a main portion provided in a direction substantially perpendicular to an optical path of the excitation light and including a slit hole; and a sidewall portion extending from an end portion of the main portion and inclined toward an upstream side in an optical path direction of the excitation light, wherein the diffracted light removal slit is provided between the light source unit and the excitation light reflector for removing diffracted light of the excitation light, wherein the sidewall portion is provided at least on an end portion side of the main portion where the reflected light exists, and does not block the reflected light. a dielectric member; a metal film adjacent to an upper surface of the dielectric member; and a sample solution holding member arranged on an upper surface of the metal film, wherein the sensor chip is an excitation light reflector, and wherein the optical sample detection system emits excitation light from the light source unit and further determines a position of the sensor chip using reflected light of the excitation light reflected at the excitation light reflector, wherein the optical sample detection system further comprises an excitation light detection unit that detects, out of the reflected light, reflected light traveling in a predetermined direction and a fluorescent detection unit, wherein the light source unit is part of a light irradiation unit, wherein a conveyance stage is actuated to move the sensor chip to a measurement position, wherein the measurement position represents a position where the excitation light irradiation unit irradiates the sensor chip with the excitation light and the fluorescence detection unit detects the fluorescence generated with the irradiation, characterised in that: a diffracted light removal slit is provided, wherein the diffracted light removal slit comprises: a main portion provided in a direction substantially perpendicular to an optical path of the excitation light and including a slit hole; and a sidewall portion extending from an end portion of the main portion and inclined toward an upstream side in an optical path direction of the excitation light, wherein the diffracted light removal slit is provided between the light source unit and the excitation light reflector for removing diffracted light of the excitation light, wherein the sidewall portion is provided at least on an end portion side of the main portion where the reflected light exists, and does not block the reflected light. a diffracted light removal slit is provided, wherein the diffracted light removal slit comprises: a main portion provided in a direction substantially perpendicular to an optical path of the excitation light and including a slit hole; and a sidewall portion extending from an end portion of the main portion and inclined toward an upstream side in an optical path direction of the excitation light, wherein the diffracted light removal slit is provided between the light source unit and the excitation light reflector for removing diffracted light of the excitation light, wherein the sidewall portion is provided at least on an end portion side of the main portion where the reflected light exists, and does not block the reflected light. a main portion provided in a direction substantially perpendicular to an optical path of the excitation light and including a slit hole; and a sidewall portion extending from an end portion of the main portion and inclined toward an upstream side in an optical path direction of the excitation light, wherein the diffracted light removal slit is provided between the light source unit and the excitation light reflector for removing diffracted light of the excitation light, wherein the sidewall portion is provided at least on an end portion side of the main portion where the reflected light exists, and does not block the reflected light.