PROBLEM TO BE SOLVED: To provide a vial adapter which can fit vials of a plurality of sizes without interposing a separate member between the vial adapter and the vial and can be easily mounted on the vials of the plurality of sizes.SOLUTION: A vial adapter disclosed in the invention includes: an adapter body having a connecting part connected to a medical device and a first fitting peripheral wall fitted to a head part of a first vial and a second fitting peripheral wall fitted to a head part of a second vial having a diameter dimension smaller than the head part of the first vial. The second fitting peripheral wall is mounted on the adapter body on a radially inner side of the first fitting peripheral wall in a manner to be slidable along an axial direction of the first fitting peripheral wall.
