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 tube 5 connected with a vent hole 223 bored at the adaptor body 2; a hydrogen peroxide absorbing substance 6 installed in a lumen of the tube 5; and a seal ring 4 which secures air-tightness between it and the vent valve. The hydrogen peroxide absorbing substance 6 is a substance based on cellulose. When gas containing hydrogen peroxide is introduced into a sterilization bath when sterilizing the endoscope with gaseous hydrogen peroxide plasma, this gas flows to inside of the endoscope through the lumen of the tube 5. At this time, the hydrogen peroxide contained in the gas is absorbed by the hydrogen peroxide absorbing substance 6.