To solve a problem that, since an inspection window mounted on a mixer drum in a conventional technology includes a frame, a seal plate and a slide plate, the inspection window is used for the mixer drum, a shape of the frame is complicated, and an outer frame is mounted by the welding, work is troublesome and cannot be performed outdoors in a rainy day.

An inspection window to be mounted on a conveyor, a hopper or the like includes a window frame body having a bottom plate with a locking piece being formed on a lower side of a cylindrical body, a packing provided on the bottom plate, a pressing frame body in which a mounting guide pin is projected, and lid holding tools are provided on both sides, and a lid having a mounting part to be attachably/detachably mounted on the lid holding tools on both sides of the lid body.