To provide an endoscope adapter which prevents gaseous hydrogen peroxide from entering from a vent valve of an endoscope to the inside.

This endoscope adapter 1 comprises: a cap-like adaptor body part 2 to be mounted to the vent valve in the case of opening the vent valve for the endoscope; a filter 3 installed so as to cover a vent hole 223 bored at the adaptor body 2; and a seal ring 4. The filter 3 contains a substance based on cellulose as a substance which can absorb hydrogen peroxide and absorbs the hydrogen peroxide contained in gas flowing in from the vent hole 223. The filter 3 is provided with: a hydrogen peroxide absorbing layer containing the substance based on the cellulose; and a water-repelling layer which permeates gas and cuts off liquid.