A light irradiation device of the present invention includes: a light source that emits radiant light to an irradiation object a reflecting member that reflects the radiant light emitted from the light source toward the irradiation object and an irradiation member that includes at least two irradiation surfaces for transmitting the radiant light reflected from the reflecting member and irradiating the irradiation object with the radiant light. The light source emits the radiant light from one end side of the irradiation surfaces of the irradiation member, and the reflecting member includes: a first reflecting area that reflects the radiant light emitted from the light source toward a first irradiation surface of the irradiation member a second reflecting area that reflects the radiant light emitted from the light source and indirectly reaches the second reflecting area, toward a second irradiation surface of the irradiation member and a third reflecting area that guides the radiant light to the second reflecting area. Therefore, the light irradiation device and a light irradiation treatment and prevention device, which can irradiate a plurality of irradiation surfaces with the radiant light using one light source, can be configured.
