An indwelling needle assembly comprises an inner needle having a sharp needle tip at the end, an inner needle hub secured to the proximal end of the inner needle, a hollow outer needle into which the inner needle is inserted, an outer needle hub which is secured to the proximal end of the outer needle and which has a projection at the proximal end, and a protector which is elastic, which has a connection member with a projecting part engageable with the projection, and which is detachably connected to the outer needle hub by the engagement of the projecting part with the projection. The projection of the outer needle hub comprises a sliding surface which is tilted relative to the axis of the inner needle and on which the projecting part is slidingly moved. The connection member can take a first state in which the projecting part is engaged with the projection and a second state in which the projecting part is disengaged from the projection. When the connection member is changed from the first state to the second state, the projecting part is slidingly moved along the sliding surface of the projection.
