An indwelling needle assembly, which is a medical instrument, has a hollow outer needle, an outer needle hub fixed to the base end of the outer needle, an inner needle inserted in the outer needle, an inner needle hub fixed to the base end of the inner needle, and a protector. The outer needle hub has a step section functioning as a speed reducing means. The speed reducing means reduces the speed, in the direction of the axis of the flow path of a main pipe, of a portion of liquid flowing in the flow path, the portion being a portion flowing along that portion of the inner surface of the main pipe which is located on that side of the main pipe on which a branch flow path is located. The step section has an opening formed in the inner surface of the main pipe. The opening is located at a position corresponding to a front end opening of the branch flow path, and the step section forms a space between the opening and the front end opening of the branch flow path.
