A device for origin of light absorbance used in an optical measuring instrument for measuring light absorption characteristics of a light diffusion sample. The device includes a light transmission terminal configured to irradiate a measurement light onto the light diffusion sample at a point of incidence of the light diffusion sample and a light diffusion plate through the interior of which the measurement light can be diffused and transmitted. A light receiving terminal is configured to receive the measurement light from the light diffusion sample for measurement at a point of detection of the light diffusion sample that is separate from the point of incidence. Optionally, the device can include a cleaning unit that cleans the light transmission terminal and light receiving terminal.