An optical sensor suitable for a detection target (T) that emits radiation light only when irradiated with excitation light of a predetermined wavelength, the radiation light being different from the excitation light, the optical sensor comprising: a light source that irradiates the excitation light; a photodetector that is capable of detecting the radiation light emitted from the detection target (T) excited by the excitation light; a single light guide unit that is capable of guiding the excitation light to the detection target (T) and is capable of guiding the radiation light to the photodetector ; and a light-blocking region that is capable of preventing the excitation light from reaching the photodetector before reaching the detection target, wherein: the light guide unit is a block-shaped light guide that is capable of transmitting the excitation light and the radiation light, the light source and the photodetector are disposed at one end of the light guide , and the light-blocking region is formed by a single step in the one end of the light guide unit between a first portion of the light guide on the side of the light source through which light from the light source is transmitted in the light guide , a second portion of the light guide on the side of the photodetector through which light from the light guide is transmitted to the photodetector , and a standing surface joining the first and second portions, whereby the first portion, the second portion, and the standing surface are contiguous with each other and define the single step from the first portion to the second portion in a direction away from the light source . a light source that irradiates the excitation light; a photodetector that is capable of detecting the radiation light emitted from the detection target (T) excited by the excitation light; a single light guide unit that is capable of guiding the excitation light to the detection target (T) and is capable of guiding the radiation light to the photodetector ; and a light-blocking region that is capable of preventing the excitation light from reaching the photodetector before reaching the detection target, wherein: the light guide unit is a block-shaped light guide that is capable of transmitting the excitation light and the radiation light, the light source and the photodetector are disposed at one end of the light guide , and the light-blocking region is formed by a single step in the one end of the light guide unit between a first portion of the light guide on the side of the light source through which light from the light source is transmitted in the light guide , a second portion of the light guide on the side of the photodetector through which light from the light guide is transmitted to the photodetector , and a standing surface joining the first and second portions, whereby the first portion, the second portion, and the standing surface are contiguous with each other and define the single step from the first portion to the second portion in a direction away from the light source . the light guide unit is a block-shaped light guide that is capable of transmitting the excitation light and the radiation light, the light source and the photodetector are disposed at one end of the light guide , and the light-blocking region is formed by a single step in the one end of the light guide unit between a first portion of the light guide on the side of the light source through which light from the light source is transmitted in the light guide , a second portion of the light guide on the side of the photodetector through which light from the light guide is transmitted to the photodetector , and a standing surface joining the first and second portions, whereby the first portion, the second portion, and the standing surface are contiguous with each other and define the single step from the first portion to the second portion in a direction away from the light source . <EOS>