An optical scanning apparatus capable of properly correcting a main scanning scale factor. Upstream and downstream laser-light detection sensors of the optical scanning apparatus each generate a signal when first or second light beam passes through each of slits formed in the sensors. Based on signals, a scanning time required for each light beam to scan between the sensors is measured. A correction coefficient is decided by a correction value calculation unit based on a difference between wavelengths of the first and second light beams, a scanning time difference is calculated by a comparison unit based on the correction coefficient and scanning times of the light beams, and the downstream laser-light detection sensor is rotated by a sensor rotation drive unit to make the scanning time difference zero, whereby the slits are made parallel to each other.