A needle assembly (1) includes a cover (5)(7) provided to be movable to a first position where a cannula (2)(3) is exposed and a second position where a distal end projects further than the cannula. In the cover, circumferential-direction locking sections (25)(43) or front-back-direction grooves (24)(42) continuing to the circumferential-direction locking sections or engaging protrusions (5a)(33a) movable along the circumferential-direction locking sections and the front-back-direction grooves are provided. In a state in which the cover is located in the first position and the engaging protrusions are housed in the circumferential-direction locking sections, movement in a front-back direction of the cover is blocked. When the engaging protrusions move along the circumferential-direction locking sections and move to the front-back-direction grooves, the engaging protrusions become movable along the front-back-direction grooves and allow the movement to the second position of the cover. Projecting sections to the outside are reduced as much as possible and movement of the cover due to a malfunction is prevented.
