To provide a highly-efficient light irradiation device by providing an optical system capable of efficiently taking an outgoing beam into an integrator from a light source composed of a plurality of LEDs.

A light irradiation device 1 comprises: a light source unit 11 including a plurality of LEDs 21; a first optical system 13 which collimates respective outgoing beams from the plurality of LEDs 21; a second optical system 15 on which the outgoing beams from the first optical system 13 are incident; a third optical system 17 on which the outgoing beams from the second optical system 15 are incident; and a lens array integrator 19 on which the outgoing beams from the third optical system 17 are incident. The third optical system 17 is an image-side telecentric optical system which projects the outgoing beams from the second optical system 15 on an incident surface 19S of the lens array integrator 19. The incident surface 19S is positioned at an imaging position of the third optical system 17. A focal position on the third optical system 17 side of the second optical system 15 matches a focal position on the second optical system 15 side of the third optical system 17.