To detect presence of a liquid and locate the boundary face of the liquid accurately without touching the liquid even in the case of a liquid having high optical transmittance by utilizing the diffraction effect of light beam on the boundary face of the liquid.

An infrared light emitting element 1 for emitting infrared rays at an angle other than 90° toward the boundary face of a liquid to be detected contained in a light transmitting container 10 is disposed on the output of the container 10. An infrared light receiving element 2 is disposed on the extension in the incident direction of infrared rays on the outside of the container 10 in order to detect variation in the quantity of receiving light caused by diffraction of incident light on the boundary face. At least one of the light emitting element 1 or the light receiving element 2 has directivity.