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