An objective of the present invention is to provide a technique for reducing measurement errors when measuring specimen using light. An aspect of an optical measurement method according to the present invention: acquires relationship data that describes a relationship between an intensity of reflection light when irradiating light onto a specimen and a size of the specimen; and acquires the size of the specimen using the relationship data and the intensity of the reflection light. Another aspect of an optical measurement method according to the present invention subtracts a component due to an inclination of a vessel of a specimen from a detection signal representing an intensity of reflection light when irradiating light onto the specimen, thereby correcting the inclination of the vessel.