At least one exemplary embodiment is directed to a scanning optical device includes an incidence optical system that causes the principal rays of all of a plurality of light fluxes emitted from a light source element comprising a plurality of light-emitting parts on the same substrate to impinge upon the same deflecting surface of a light deflector at different angles in a plane orthogonal to a main scanning plane, and be deflected to form a latent image on a plurality of different photosensitive scanning surfaces, thereby achieving a scanning optical device and an image forming apparatus that are small in size and inexpensive.