An indwelling needle assembly (10, 96, 140, 240, 280) which includes an inner needle unit (12, 114, 152, 272, 302) and an outer needle unit, the inner needle unit (12, 114, 152, 272, 302) including an inner needle having a needle tip, the outer needle unit including an outer needle hub, wherein the inner needle is inserted into the outer needle hub, a safety mechanism part (22, 98, 142, 212, 226, 244, 282) is externally mounted about the inner needle and is configured to protect the needle tip of the inner needle when the inner needle is pulled out of the outer needle hub, the safety mechanism part (22, 98, 142, 212, 226, 244, 282) includes at least one urging part (58, 60, 102, 200, 260, 294a, 294b) urged in a direction of approaching the inner needle and a block part (82, 174, 254, 296, 320) that is in contact with the urging part (58, 60, 102, 200, 260, 294a, 294b) such that the urging part (58, 60, 102, 200, 260, 294a, 294b) is kept remote from the inner needle, the block part (82, 174, 254, 296, 320) includes an inner-needle engaging part (84, 158, 234, 296a) that is engaged with the inner needle in an axial direction and a peripheral wall part on which the urging part (58, 60, 102, 200, 260, 294a, 294b) is held in contact and being disposed about the inner needle with a radial space therebetween so that the peripheral wall part is radially spaced away from the inner needle, and the urging part (58, 60, 102, 200, 260, 294a, 294b) and the block part (82, 174, 254, 296, 320) are movable relative to each other in a lengthwise direction of the inner needle, the indwelling needle assembly (10, 96, 140, 240, 280) being characterized in that: the safety mechanism part (22, 98, 142, 212, 226, 244, 282) further includes an outer-needle-hub engaging part (54, 118, 194, 288) that is engaged with an outer surface of the outer needle hub when the inner needle unit (12, 114, 152, 272, 302) and the outer needle unit are attached, and an engaging-part opposing wall (55, 130, 202, 222, 258, 298b) that is positioned in opposition to the outer-needle-hub engaging part (54, 118, 194, 288) and limits displacement of the outer needle hub in a direction of disengagement from the outer-needle-hub engaging part (54, 118, 194, 288) by being in contact with the outer needle hub, and the outer-needle-hub engaging part (54, 118, 194, 288) and the engaging-part opposing wall (55, 130, 202, 222, 258, 298b) are integrally provided to a main body (52, 100) of the safety mechanism part (22, 98, 142, 212, 226, 244, 282), and therefore relative rotation of the outer-needle-hub engaging part (54, 118, 194, 288) and the engaging-part opposing wall (55, 130, 202, 222, 258, 298b) around the inner needle is restricted; at least one of the outer-needle-hub engaging part (54, 118, 194, 288) and the engaging-part opposing wall (55, 130, 202, 222, 258, 298b) is configured to displace integrally with the urging part (58, 60, 102, 200, 260, 294a, 294b); and by the inner needle being pulled out of the outer needle hub, the block part (82, 174, 254, 296, 320) engaged with the inner needle by the inner-needle engaging part (84, 158, 234, 296a) is configured to move relative to the urging part (58, 60, 102, 200, 260, 294a, 294b) to be in a state of non-contact, while the urging part (58, 60, 102, 200, 260, 294a, 294b) is configured to displace in the direction of approaching the inner needle such that the needle tip of the inner needle is prevented from protruding from the safety mechanism part (22, 98, 142, 212, 226, 244, 282) and engagement of the outer needle hub with the safety mechanism part (22, 98, 142, 212, 226, 244, 282) is allowed to be released. the safety mechanism part (22, 98, 142, 212, 226, 244, 282) further includes an outer-needle-hub engaging part (54, 118, 194, 288) that is engaged with an outer surface of the outer needle hub when the inner needle unit (12, 114, 152, 272, 302) and the outer needle unit are attached, and an engaging-part opposing wall (55, 130, 202, 222, 258, 298b) that is positioned in opposition to the outer-needle-hub engaging part (54, 118, 194, 288) and limits displacement of the outer needle hub in a direction of disengagement from the outer-needle-hub engaging part (54, 118, 194, 288) by being in contact with the outer needle hub, and the outer-needle-hub engaging part (54, 118, 194, 288) and the engaging-part opposing wall (55, 130, 202, 222, 258, 298b) are integrally provided to a main body (52, 100) of the safety mechanism part (22, 98, 142, 212, 226, 244, 282), and therefore relative rotation of the outer-needle-hub engaging part (54, 118, 194, 288) and the engaging-part opposing wall (55, 130, 202, 222, 258, 298b) around the inner needle is restricted; at least one of the outer-needle-hub engaging part (54, 118, 194, 288) and the engaging-part opposing wall (55, 130, 202, 222, 258, 298b) is configured to displace integrally with the urging part (58, 60, 102, 200, 260, 294a, 294b); and by the inner needle being pulled out of the outer needle hub, the block part (82, 174, 254, 296, 320) engaged with the inner needle by the inner-needle engaging part (84, 158, 234, 296a) is configured to move relative to the urging part (58, 60, 102, 200, 260, 294a, 294b) to be in a state of non-contact, while the urging part (58, 60, 102, 200, 260, 294a, 294b) is configured to displace in the direction of approaching the inner needle such that the needle tip of the inner needle is prevented from protruding from the safety mechanism part (22, 98, 142, 212, 226, 244, 282) and engagement of the outer needle hub with the safety mechanism part (22, 98, 142, 212, 226, 244, 282) is allowed to be released.