To hold surely a transparent plate in an outer frame body and perform assembly work thereof safely and properly, for an observation window which is provided in a door or outer wall of a distribution board, switchboard, etc.

An observation window is composed of: a window opening 2 provided in a wallboard 8; an outer frame body 3 arranged on the outside of the window opening 2 and made of a rigid body; an inner frame body 4 arranged on the inside of the window opening 2 and made of a rigid body; an intermediate frame body 5 arranged between the outer frame body 3 and the inner frame body 4 and made of a rigid body; a transparent plate 6 arranged in the window opening 2 and fixedly sandwiched in advance by the outer frame body 3 and the intermediate frame body 5; and a waterproof packing 7 which performs mutual sealing of a periphery of the window opening 2 of the wallboard 8, the outer frame body 3, and the inner frame body 4 by fastening the outer frame body 3, including the intermediate frame body 5 and the transparent plate 6, and the inner frame body 4. Especially, the intermediate frame body 5 made of a rigid body is provided while the transparent plate 6 is fixedly sandwiched in advance by the outer frame body 3 similarly made of a rigid body.