A needle assembly including: a needle projecting from a needle hub; and a needle tip cover having a needle hole into which the needle is inserted, the needle tip cover being externally provided about the needle so as to be movable in a needle axis direction and configured to cover a needle tip by being moved to a distal end side beyond the needle tip, the needle assembly being characterized in that: the needle hole has an inside diameter dimension larger than an outside diameter dimension of the needle; a fixed-side guide is fixedly provided to the needle; a moving-side guide is integrally provided to the needle tip cover; characterized in that the moving-side guide is engaged with the fixed-side guide so as to provide a guide mechanism to guide the needle tip cover from a proximal end side of the needle toward the distal end side thereof while keeping the needle tip cover from contacting the needle and, at a position beyond the needle tip, to deviate the needle tip cover in a direction in which the needle hole goes away from a center axis of the needle so as to avoid contact of the needle tip with the needle hole while guiding the needle tip cover to a position where the needle tip cover covers the needle tip; the fixed-side guide includes a guiding groove extending in the needle axis direction; the moving-side guide includes an elongated guiding rod to be guided by the guiding groove; wherein an axially medial portion of the guiding rod is provided with a stepped part which is bent into a roughly crank, whereby with the stepped part interposed in between, the distal end side of the guiding rod is positioned below the proximal end side thereof so as to approach the needle hub; wherein opposite parts of the guiding grove include stepped parts at their axially medial portion; wherein when the moving side guide is displaced to the axially distal end side, the needle tip cover moves to the position beyond the needle tip of the needle, wherein a center axis of the needle and a center axis of the needle hole are coaxially positioned; wherein the needle tip cover is configured to be moved in the direction in which the needle hole goes away from the center axis of the needle by the guiding rod being guided and moved with respect to the guiding groove toward the distal end side of the needle in the needle axis direction so that the stepped part of the guiding rod reaches a position of the stepped parts of the guiding groove. the needle hole has an inside diameter dimension larger than an outside diameter dimension of the needle; a fixed-side guide is fixedly provided to the needle; a moving-side guide is integrally provided to the needle tip cover; characterized in that the moving-side guide is engaged with the fixed-side guide so as to provide a guide mechanism to guide the needle tip cover from a proximal end side of the needle toward the distal end side thereof while keeping the needle tip cover from contacting the needle and, at a position beyond the needle tip, to deviate the needle tip cover in a direction in which the needle hole goes away from a center axis of the needle so as to avoid contact of the needle tip with the needle hole while guiding the needle tip cover to a position where the needle tip cover covers the needle tip; the fixed-side guide includes a guiding groove extending in the needle axis direction; the moving-side guide includes an elongated guiding rod to be guided by the guiding groove; wherein an axially medial portion of the guiding rod is provided with a stepped part which is bent into a roughly crank, whereby with the stepped part interposed in between, the distal end side of the guiding rod is positioned below the proximal end side thereof so as to approach the needle hub; wherein opposite parts of the guiding grove include stepped parts at their axially medial portion; wherein when the moving side guide is displaced to the axially distal end side, the needle tip cover moves to the position beyond the needle tip of the needle, wherein a center axis of the needle and a center axis of the needle hole are coaxially positioned; wherein the needle tip cover is configured to be moved in the direction in which the needle hole goes away from the center axis of the needle by the guiding rod being guided and moved with respect to the guiding groove toward the distal end side of the needle in the needle axis direction so that the stepped part of the guiding rod reaches a position of the stepped parts of the guiding groove.