An optical device includes: a light source including a plurality of light emitting spots that output laser beams, respectively; a separating unit that separates each of the laser beams output from the plurality of light emitting spots into a monitor beam and a scanning beam; a light-quantity measuring unit that measures a light quantity of the monitor beam; a storage unit in which respective drive currents with which the plurality of light emitting spots of the light source output a prescribed light quantity of laser beams are stored in advance; a light-source control unit that drives the light source with the drive currents stored in the storage unit and causes the plurality of light emitting spots to output the laser beams; and a determining unit that determines whether the light source operates properly on the basis of the light quantity measured by the light-quantity measuring unit.