A data acquisition apparatus includes an illumination device, a first beam splitter, a measurement unit, and a photodetector. A measurement optical path and a reference optical path are positioned between the illumination device and the photodetector. In the first beam splitter, light traveling in a first direction and light traveling in a second direction are generated from incident light. The measurement optical path is positioned in the first direction, the reference optical path is positioned in the second direction, and the measurement unit is disposed on the measurement optical path. In the optical surface of the first beam splitter, an incident position of light emitted from the illumination device changes with time, and the angle formed by light propagating through the measurement optical path and the optical axis of the measurement optical path changes with change in the incident position.