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