An image forming apparatus, including: a light source configured to emit a light beam; a rotary polygon mirror having a plurality of reflection surfaces configured to deflect the light beam; a beam detector configured to receive the light beam to output a pulse; a pulse interval measurement unit configured to measure a pulse interval of pulses output from the beam detector; a reflection surface identification unit configured to identify each of the plurality of reflection surfaces; a storage portion configured to store a reference pulse interval of each of the plurality of reflection surfaces; a correction amount calculation unit configured to calculate a correction amount based on the pulse interval and the reference pulse interval for each of the plurality of reflection surfaces; and a light source control unit configured to control the light source based on the correction amount.