Patent ID: 7087037

Claim:
An injector comprising a barrel for mounting an injection needle thereon and a plunger with a gasket joined to a tip end thereof and held in intimate sliding contact with an inner wall surface of the barrel, said gasket being of substantial plate-shape, an entire side surface thereof being a curved surface having a convex-shaped are rising in the direction of the inner wall surface of the barrel and extending along an axis direction of the plunger, and being held in intimate contact with the inner wall surface of the barrel at a portion of the curved surface thereof, said gasket being joined to a mount base, the mount base being disposed on a tip end of the plunger and being smaller in diameter than the tip end of the plunger, with a gap being left, the gap allowing said gasket to be bendable toward the tip end of the plunger, said gasket having an annular groove defined in a surface thereof remote from said plunger and larger in diameter than said mount base, said gasket having a bendable portion, which bends about a bottom of said annular groove in such a manner that the bendable portion falls in a direction radially outwardly with respect to said gasket when the gasket is held in intimate contact with an inner wall surface of the barrel and slides toward a tip end of the barrel, and bends about the bottom of the annular groove in such a manner that the bendable portion falls in a direction radially inwardly with respect to the gasket when the gasket is held in intimate contact with a inner wall surface of the barrel and slides toward a back end of the barrel, wherein the curved surface of said bendable portion includes a portion which is held in intimate contact with the inner wall surface of the barrel when the gasket is bent, the portion of the curved surface which is held with the inner wall surface of the barrel when the gasket is bent being formed with a radius R 2 , and wherein a distance R 1 between the bottom of said annular groove and a crest of said curved surface is smaller than the radius R 2 .