An optical measurement device is adapted to measure characteristics of a measurement targeted fluid based on a transmitted inspection light. The device comprises a cell having an internal space for accommodating the measurement targeted fluid to flow therein and having a pair of opposing through holes for transmitting the inspection light, each of the through holes sealed with a transparent member. The device includes a projection optical system member having a port for projecting the inspection light and a light-receiving optical system member having a port for receiving the inspection light transmitted through the internal space. A base member is provided for supporting the cell, the projection optical system member, and the light-receiving optical system member in a configuration with gaps between the cell and the respective optical system members to accommodate movement in a predetermined range along a direction perpendicular to an optical axis of the inspection light.