An electrostatic latent image is formed by exposing the same exposure point on a photosensitive drum to a laser beam having an emission wavelength of 420 nm and a laser beam having an emission wavelength of 780 nm. The exposure is performed in a pulse-height modulation manner by changing a total output of these two laser beams to write the electrostatic latent image while changing an output ratio between the two laser beams depending on the total output.