This light irradiation device is provided with a light source for emitting radiation light to a subject to be irradiated, a reflection member for reflecting the radiation light emitted from the light source towards the subject to be irradiated, and an irradiation member having at least two irradiation surfaces for transmitting the radiation light reflected by means of the reflection member and irradiating the subject to be irradiated with the radiation light. The light source emits the radiation light from one of the edges of the irradiation surface of the irradiation member. The reflection member is provided with a first reflection region for reflecting the radiation light from the light source off to the first irradiation surface of the irradiation member, a second reflection region for reflecting the radiation light that was emitted from the light source and that intermittently reached the second reflection region off to the second irradiation surface of the irradiation member, and a third irradiation region for guiding the radiation light to the second reflection region. As a consequence, it is possible to obtain a light irradiation device and a light irradiation treatment and prevention device capable of irradiating a plurality of irradiation surfaces by using one light source.Le dispositif dirradiation lumineuse de linvention est équipé : dune source de lumière qui émet en rayonnement une lumière de radiation sur un sujet à irradier dun élément réfléchissant qui réfléchit la lumière de radiation rayonnant à partir de la source de lumière sur le sujet à irradier et dun élément dirradiation qui possède au moins deux faces dirradiation qui irradient le sujet à irradier en faisant passer au travers la lumière de radiation ainsi réfléchie par lélément réfléchissant. La source de lumière émet en rayonnement la lumière de radiation à partir dun côté extrémité de la face dirradiation de lélément dirradiation. Lélément de réflexion est équipé : dune première région de
