To adapt an optical writing apparatus using a VCSEL as a light source device to a multistage image forming linear speed while suppressing the range of light emission output.

The 40-channel VCSEL is used as the light source device and, for example, when a process linear speed is changed to 0.95 times as large as a maximum value Vmax, control is so performed that one beam from each of both ends of channels (both ends in a sub-scanning direction), i.e. two beams are not emitted. The number of beams (the number of CHs) in use is 38. Consequently, an interlaced scan can be made without causing overlap of dots.