To provide a structure useful as a light irradiation device for cleaning a template in a nanoimprint device wherein temperature control is severe by suppressing heat generation in a light emission window, in the light irradiation device including an excimer lamp, a casing storing the excimer lamp, and the light emission window for emitting excimer light from the casing to the outside.

The light emission window includes a central light emission part, and a peripheral installation flange part. In the light emission part, the rear face on the excimer lamp side is recessed as compared to the flange part rear face, and a wall thickness of the light emission part is thinner than a wall thickness of the flange part.