A needle assembly 1 includes a hub 4 retaining a cannula 3, a protective unit 5 advancing and retracting to surround the forward end of the cannula 3, and a cylindrical portion 6 surrounding the outer periphery of the protective unit 5； the protective unit 5 has a construction in which a protective cover 7 protruding from and retracting into the cylindrical portion 6 is formed integrally with a spring portion 8 so as to be in series therewith, with the hub 4 and the cylindrical portion 6 being coupled together to form a hub member. Further, the protective cover 7 is provided with an engagement protrusion 24, and the hub member is provided with a guide means G； when the protective cover 7 is retracted from a pre-use state ( Fig. 4(a) ) in which it is situated at an advanced position, the engagement protrusion 24 moves backwards while rotating and in contact with an inclined surface 14a ( Fig. 4(b) )； when the protective cover 7 advances from a use state ( Fig. 4(c) ) in which it is situated at a retracted position, the engagement protrusion 24 moves forwards to be fixed to a post-use retaining portion B to be thereby prevented from retracting. The needle assembly can be produced at low cost and used safely.
