An indwelling needle assembly (10, 110, 160) comprising: an inner needle having an inner needle hub on a proximal end side thereof; an outer needle having an outer needle hub on a proximal end side thereof, the outer needle receiving the inner needle being inserted from a proximal end side of the outer needle hub; a needle tip protector (22, 111, 162) attached to the inner needle, the needle tip protector (22, 111, 162) configured to move to a distal end of the inner needle so as to cover a needle tip of the inner needle; and a connection mechanism connecting the proximal end side of the outer needle hub to a distal end side of the needle tip protector (22, 111, 162),: a connection member (94, 140, 164) is provided extending out from the needle tip protector (22, 111, 162) to an inside of the outer needle hub while being abutted against an inner face of the outer needle hub so as to restrict movement of the outer needle hub relative to the needle tip protector (22, 111, 162) and to keep a connected state of the outer needle hub to the needle tip protector (22, 111, 162) by the connection mechanism, a disconnection mechanism is provided and configured to move the connection member (94, 140, 164) to an inner peripheral side of the outer needle hub so as to release the connected state when the inner needle is extracted from the outer needle, characterized by a proximal end side of the connection member (94, 140, 164) is housed between opposite faces of a pair of guide walls (70,118) provided at the needle tip protector (22,111,162), the connection member (94, 140, 164) being assembled as movable in a diametrical direction relative to the needle tip protector (22,111,162) while being guided by the pair of guide walls (70,118) so that the connection member (94, 140, 164) is configured to be moved to the inner peripheral side of the outer needle hub. an inner needle having an inner needle hub on a proximal end side thereof; an outer needle having an outer needle hub on a proximal end side thereof, the outer needle receiving the inner needle being inserted from a proximal end side of the outer needle hub; a needle tip protector (22, 111, 162) attached to the inner needle, the needle tip protector (22, 111, 162) configured to move to a distal end of the inner needle so as to cover a needle tip of the inner needle; and a connection mechanism connecting the proximal end side of the outer needle hub to a distal end side of the needle tip protector (22, 111, 162),: a connection member (94, 140, 164) is provided extending out from the needle tip protector (22, 111, 162) to an inside of the outer needle hub while being abutted against an inner face of the outer needle hub so as to restrict movement of the outer needle hub relative to the needle tip protector (22, 111, 162) and to keep a connected state of the outer needle hub to the needle tip protector (22, 111, 162) by the connection mechanism, a disconnection mechanism is provided and configured to move the connection member (94, 140, 164) to an inner peripheral side of the outer needle hub so as to release the connected state when the inner needle is extracted from the outer needle, characterized by a proximal end side of the connection member (94, 140, 164) is housed between opposite faces of a pair of guide walls (70,118) provided at the needle tip protector (22,111,162), the connection member (94, 140, 164) being assembled as movable in a diametrical direction relative to the needle tip protector (22,111,162) while being guided by the pair of guide walls (70,118) so that the connection member (94, 140, 164) is configured to be moved to the inner peripheral side of the outer needle hub. a connection member (94, 140, 164) is provided extending out from the needle tip protector (22, 111, 162) to an inside of the outer needle hub while being abutted against an inner face of the outer needle hub so as to restrict movement of the outer needle hub relative to the needle tip protector (22, 111, 162) and to keep a connected state of the outer needle hub to the needle tip protector (22, 111, 162) by the connection mechanism, a disconnection mechanism is provided and configured to move the connection member (94, 140, 164) to an inner peripheral side of the outer needle hub so as to release the connected state when the inner needle is extracted from the outer needle, characterized by a proximal end side of the connection member (94, 140, 164) is housed between opposite faces of a pair of guide walls (70,118) provided at the needle tip protector (22,111,162), the connection member (94, 140, 164) being assembled as movable in a diametrical direction relative to the needle tip protector (22,111,162) while being guided by the pair of guide walls (70,118) so that the connection member (94, 140, 164) is configured to be moved to the inner peripheral side of the outer needle hub.