To provide a light irradiation device that secures safety for preventing mutual contamination between subjects by safely bleaching teeth in a short time or inflammation, etc., due to a heat action in a dental pulp or circumferential gum, etc., without requiring a mouth to be opened for a long time during the bleaching or making a tray to thereby complete the bleaching on the same day.

The mouthpiece type light irradiation device includes a temperature sensor in a laser or LED irradiation part, removes heat generated in a light source by a cooling part in the irradiation device, and controls an increase in temperature in a tooth due to the light irradiation. A light permeable gel pack is mounted on the light irradiation device, which is mounted on the device, contains aqueous gel with low hardness to mold teeth, and is easily exchanged.