A medical instrument configured to cover at least a mouth section of a medical container that includes a container body with the mouth section at a distal end and a sealing member that seals the mouth section, includes: a cap comprising a cylindrical body； an adapter； and a proximal-end side structure body. The medical device is configured such that, when screw-engagement between a second cap-side screw-engaged portion and a proximal-end side screw-engaged portion is released by rotating the cap around a center axis, the adapter moves in the proximal-end direction while being prevented from rotating by a guide portion, thereby allowing an adapter-side screw-engaged portion to be detached from a first cap-side screw-engaged portion, and further allowing a hollow needle to penetrate the sealing member so that an inside of the container body communicates with an outside via the hollow needle.
