To provide medication to a patient in a short time with liquid medicine having an adjusted dissolution speed, by utilizing a medication instrument which a secondary path is formed in and a first exit branched from a first flow path is connected to, installing a path selecting means at the branch, and installing a check valve at a third path connected to a container vessel and the second inlet.

When a liquid medicine is medicated with dissolving in a diluting solution, a buffer chamber 2 is mounted with an adapter 3, and an elastic part of a medicine containing cartridge 11 is fixed to a needle of the adapter 3. Then, a three-way cock to communicate the first inlet 7, the first outlet 12, and a connecting part of a container 15 is opened. Thus, a diluting solution flowing from the first inlet 7 is forced to flow to the container 14 through the first outlet 12 in the direction of the adapter 3 through a connecting opening 15. Meanwhile, a liquid flowing from the first outlet 12 flows into a cartridge 11 at the adapter 3. After preparing liquid medicine by dissolving a medicine with the dilution liquid, the liquid medicine is returned to the buffer chamber 2 through the second inlet 13, and then the liquid medicine is forced to flow to an intravenous drip injection cylinder 6 through the second outlet 9.