An adapter unit (10) is used in a state mounted on a medical container (1), and is provided with a cap (7A), an adapter (30A), distal end-side structure (8A) having a distal end-side guide part (82) for guiding the adapter (30A), and a proximal end-side structure (6A) having a proximal end-side guide part (67) for guiding the adapter (30A). When a mounted state is to be obtained, the cap (7A), the adapter (30A), and the distal end-side structure (8A) are assembled to form a cap assembly (102), and the cap assembly (102) and the proximal end-side structure (6A) are then assembled. During this operation, the distal end-side guide part (82) and the proximal end-side guide part (67) collide with each other at least once, and one or both are elastically deformed, whereby one overlaps the other and the two are thereafter mutually engaged.
