An indwelling needle assembly has an inner needle having a sharp point at its tip, an inner needle hub fixed to the base end of the inner needle, a hollow outer needle in which the inner needle is inserted, and a seal section provided in the outer needle hub. In the indwelling needle assembly, the seal section has open/close means having a seal section body with a hole into which the inner needle is insertable, a shutter member for opening and closing the hole, and an urging member for urging the shutter member in the direction to close the hole.