A sealing jig comes into contact with a cylinder inner peripheral surface as a surface to be processed of a cylinder block and seals the cylinder inner peripheral surface in introducing treatment liquid to the cylinder inner peripheral surface. The sealing jig includes: a sealing member made of an expandable material and having a ring-buoy shape; a lower plate for supporting a lower side of the sealing member; and a seal base disposed so as to face the lower plate and to support an upper side of the sealing member. The sealing member is regulated by the lower plate and the seal base and expanded only in a radial direction to come into contact with the cylinder inner peripheral surface when air is introduced inward.