To provide a device for preventing pollution of an endoscope which enables easy and smooth covering of an overcoat sheath for an insertion portion of the endoscope without using any specific auxiliary unit or the like, not permitting the overcoat sheath to be pinched between the insertion portion of the endoscope and a protective cylinder at attaching/detaching when the insertion portion of the endoscope is inserted in the overcoat sheath in case the protective cylinder at attaching/detaching is in a state of being covered.

A means 28 that seals a space between an opening 24 at a basic edge of the overcoat sheath 21 and an outer surface of the insertion portion 11 of the endoscope is installed when the insertion portion 11 of the endoscope is inserted in the overcoat sheath 21, so that an air inside the overcoat sheath 21 is compressed when the insertion potion 11 of the endoscope is inserted in the overcoat sheath 21, which is swollen in the protective cylinder 30 at attaching/detaching.