Provided is an indwelling needle assembly having a novel structure with which a protection mechanism of a needle tip of an inner needle pulled out from an outer needle is maintained, increase in the size of an outer needle hub to the outer peripheral side can be avoided, and treatment work becomes easier. In the indwelling needle assembly 10, a connection member 94 is provided which extends from a needle tip protector 22 to the inside of an outer needle hub 26 and regulates movement of the outer needle hub 26 relative to the needle tip protector 22 by contacting with the inner surface of the outer needle hub 26, thereby maintaining a connection state made by a connection mechanism, and a connection release mechanism is provided which moves the connection member 94 to the inner peripheral side of the outer needle hub 26 when an inner needle 18 is pulled out from an outer needle 24, to release the connection state of the outer needle hub 26 with the needle tip protector 22.
