An indwelling needle assembly includes a stylet having a sharp needlepoint at an end thereof, a stylet hub secured to a base end of the stylet, a hollow outer needle in which the stylet is inserted, an outer needle hub secured to a base end of the outer needle, an opening formed at the base end or at a side of the outer needle hub to communicate with the inner cavity of the outer needle, a seal fitted onto the outer needle hub and in which a hole or slit is provided for insertion of the stylet therein, the hole or slit being formed to become closed when the inserted stylet is extracted, an operating part installed on the outer needle hub to move the stylet and outer needle along the longitudinal direction when the stylet is inserted into the outer needle, and a movement suppresser for stopping or suppressing moving the stylet relative to the seal when the operating part is operated.