22 the hub 22 has an axial hole 23 disposed in fluid communication with the axial center through hole of the needle cannula 21 and a rear coupling portion 24 capped on the conical front connector 14 of the barrel 1 and engaged into the tearing groove 17 the rear coupling portion 24 has a beveled outer surface 241 disposed in contact with the connection portion 16 because the rear coupling portion 24 is capped on the conical front connector 14 of the barrel 1 and engaged into the tearing groove 17 with the beveled outer surface 241 disposed in contact with the connection portion 16 the needle assembly 2 is held positively in position and can stop leaking of the contained fluid medicine out of the barrel 1 in case the tearing groove 17 is broken accidentally therefore the hub 22 of the needle assembly 2 has the functions of 1 preventing biasing of the conical front connector 14 to cause breaking of the tearing groove 17 upon an external force and 2 stopping the contained fluid medicine from leaking out of the barrel 1 when the tearing groove 17 is broken accidentally the plunger 3 comprises a radially ribbed shank 32 a thumb rest 31 at the rear end of the shank 32 a head 33 at the front end of the shank 32 a neck 37 connected between the shank 32 and the head 33 a front extension rod 34 axially forwardly extending from the center of the front side of.