In an indwelling needle assembly 10, a safety mechanism part 22 is provided with an outer-needle-hub engaging part 54 to be engaged with an outer needle hub 26, and an engaging-part opposing wall 55. Relative rotation of the outer-needle-hub engaging part 54 and the engaging-part opposing wall 55 is restricted, and at least one of the outer-needle-hub engaging part 54 and the engaging-part opposing wall 55 displaces integrally with urging parts 58, 60. When an inner needle 18 is pulled out of the outer needle hub 26, a block part 82 engaged with the inner needle 18 through an inner-needle engaging part 84 is moved relative to the urging parts 58, 60 and forced to a non-contact state, and the urging parts 58, 60 displace in a direction approaching the inner needle 18, thereby preventing a needle tip 16 of the inner needle 18 from protruding from the safety mechanism part 22 and releasing engagement of the outer needle hub 26 with the safety mechanism part 22.
