An illumination apparatus generates an interference fringe. An input arm receives an input light beam from a light source. A splitter splits the input light beam that has passed through the input arm into a first output arm and a second output arm. A phase modulator changes a phase difference between the output light beams of the first output arm and the second output arm. A phase detector detects the phase difference between output light beams respectively output from the first output arm and the second output arm based on a return light beam generated by combining a first reflected light beam and a second reflected light beam respectively reflected by ends of the first output arm and the second output arm.