The body (11) of a light irradiation device (9) that irradiates the teeth (40) of a subject (1) with light is detachably attached to a mouth opening implement (7) that is placed in the mouth (3) and opens the mouth of the subject (1). An attachment (43) is mounted to both the left and right sides of the mouth opening implement (7) and the body (11) is mounted to the mouth opening implement (7) by means of clips (47) on the body (11) which hold the flat plates of the attachments (43) from above and below.
