A method for adjusting a multi-beam source unit including a multi-beam laser diode provided with light emitting points, and a collimator lens includes measuring a state of a position of the light emitting points with respect to a standard design line based on beam spots corresponding to the light emitting points on an image recording surface, and rotating the multi-beam laser diode about an optical axis of a scanning optical system to align the position of the light emitting points with the standard design line.