A projection optical system includes a laser diode; a laser diode module having a plurality of the laser diodes one-line arranged; a light projecting lens to adjust spread of light emitted from the respective laser diodes; and an optical deflector to deflect light passing through the light projecting lens and irradiate a target with the deflected light. The laser diodes emit light such that the light spreads at a first angle in a first direction and spreads at a second angle being wider than the first angle in a second direction being perpendicular to the first direction. The laser diode modules are disposed such that an arrangement direction thereof coincides with the first direction and they are placed along the second direction and shifted in position relative to each other in the first direction. The light projecting lenses are disposed with corresponding to the laser diode modules.