A light irradiation device (1) includes a light-source module (2), a light deflector (3), a parallel light generation unit (4), and a projecting light generation unit (5) in this order along the optical path. All of the light-collection point of the light-source module (2) on the emission side, the center mirror Or of the light deflector (3), and the focal point of the parallel light generation unit (4) on the incidence side are set at the same position P1. The mirror (31) of the light deflector (3) forward- and reverse-rotates about the axis passing through the mirror center Or to emit scanning light Ls as the reflected light and the deflected light to the parallel light generation unit (4). The parallel light generation unit (4) collimates the scanning light Ls to be parallel light that is parallel to the optical axis for emission.