An optical device includes: an irradiation unit that irradiates a part of laser beams output from a light source as a scan beam onto an irradiation target and outputs the remaining part of the laser beams as a monitor beam used to monitor a light amount of the laser beams; a measurement unit that measures a light amount of the monitor beam; a storage unit that stores a plurality of measurement results obtained by the measurement unit when the laser beams are output in a plurality of different light amounts, and the plurality of the different light amounts associated with each other; and a prediction unit that predicts the light amount of the monitor beam relative to a reference light amount of the laser beam using a plurality of the light amounts stored in the storage unit and the measurement results corresponding to the plurality of the light amounts.