A medical instrument that covers at least a mouth section of a medical container having a tubular shape and including a container body with the mouth section at a distal end and a soft sealing member that seals the mouth section, comprising: a cap including a first cap-side screw-engaged portion and a second cap-side screw-engaged portion, the first cap-side screw-engaged portion being formed of a cylindrical body and including a first screw portion formed in a spiral shape on an inner peripheral side of the cylindrical body around its center axis and the second cap-side screw-engaged portion including a second screw portion formed in a spiral shape around the center axis on a more proximal-end side than the first cap-side screw-engaged portion on an outer peripheral side of the cylindrical body or on the inner peripheral side of the cylindrical body; an adapter including an adapter-side screw-engaged portion and a hollow needle that can thrust the sealing member, the adapter-side screw-engaged portion being arranged movable in a proximal-end direction inside the cap and configured to be screw-engaged with the first cap-side screw-engaged portion; and a proximal-end side structure body including a proximal-end side screw-engaged portion to be screw-engaged with the second cap-side screw-engaged portion, a mounting section to be mounted on the container body so as to cover at least the mouth section, and a guide portion configured to guide the adapter along a moving direction and also restrict the adapter from rotating together with the cap when the adapter moves in the proximal-end direction, wherein a pitch at the first screw portion is larger than a pitch at the second screw portion, and when screw-engagement between the second cap-side screw-engaged portion and the proximal-end side screw-engaged portion is released by rotating the cap around the center axis and the cap is detached from the proximal-end side structure body, the adapter moves in the proximal-end direction while being prevented from rotating by the guide portion, thereby allowing the adapter-side screw-engaged portion to be detached from the first cap-side screw-engaged portion, and further allowing the hollow needle to thrust the sealing member so that inside of the container body communicates with outside via the hollow needle. a cap including a first cap-side screw-engaged portion and a second cap-side screw-engaged portion, the first cap-side screw-engaged portion being formed of a cylindrical body and including a first screw portion formed in a spiral shape on an inner peripheral side of the cylindrical body around its center axis and the second cap-side screw-engaged portion including a second screw portion formed in a spiral shape around the center axis on a more proximal-end side than the first cap-side screw-engaged portion on an outer peripheral side of the cylindrical body or on the inner peripheral side of the cylindrical body; an adapter including an adapter-side screw-engaged portion and a hollow needle that can thrust the sealing member, the adapter-side screw-engaged portion being arranged movable in a proximal-end direction inside the cap and configured to be screw-engaged with the first cap-side screw-engaged portion; and a proximal-end side structure body including a proximal-end side screw-engaged portion to be screw-engaged with the second cap-side screw-engaged portion, a mounting section to be mounted on the container body so as to cover at least the mouth section, and a guide portion configured to guide the adapter along a moving direction and also restrict the adapter from rotating together with the cap when the adapter moves in the proximal-end direction, wherein a pitch at the first screw portion is larger than a pitch at the second screw portion, and when screw-engagement between the second cap-side screw-engaged portion and the proximal-end side screw-engaged portion is released by rotating the cap around the center axis and the cap is detached from the proximal-end side structure body, the adapter moves in the proximal-end direction while being prevented from rotating by the guide portion, thereby allowing the adapter-side screw-engaged portion to be detached from the first cap-side screw-engaged portion, and further allowing the hollow needle to thrust the sealing member so that inside of the container body communicates with outside via the hollow needle.