An optical measurement apparatus and a calibration method, which are able to readily perform a calibration process, are provided. A recording unit 26 that records a value of a ratio of a first detection intensity when illumination light is irradiated by a measurement probe 3 to a first medium having the same refractive index as a specimen S1 to a second detection intensity when the illumination light is irradiated by the measurement probe 3 to a second medium, and a calculation unit 271 that calculates a background intensity of background light to be used in correcting returned light of the illumination light from the specimen S1, from a product of the value of the ratio recorded in the recording unit 26 and a third detection intensity when the illumination light is irradiated by the measurement probe 3 to the second medium.
