A light irradiation device includes: a light source to output diffused light; and a light guide member to guide the diffused light to an irradiation point. The light guide member includes an incidence surface which the diffused light enters, a first emission surface to emit the entered diffused light, while further diffusing the entered diffused light with a first diffusion characteristic to emit the diffused light at a first diffusion angle, and a second emission surface to emit the entered diffused light, while further diffusing the entered diffused light with a second diffusion characteristics to emit the diffused light at a second diffusion angle. The second diffusion angle provided by the second diffusion characteristic is smaller than the first diffusion angle provided by the first diffusion characteristic.