An apparatus for operating a syringe has developed comprising: an upper semi-cylindrical tube section, semi-circular shaped locking groove and lower semi-cylindrical tube section. The upper semi-cylindrical tube section is integrally formed an upper head part with a threaded hole for axial movement. The lower semi-cylindrical tube section forms a top portion, intermediate portion and end portion. A diameter (Dc) at the intermediate portion larger than the diameter (Dtip) at end portion is gradually reduced from intermediate to end portion. The length of the tapered portion is same as that portion of the constant diameter. A tip portion has formed an opened semi-circular shape to simply overlap the syringe over the cartridge for easily snapping-in and out. The syringe piston moves downward with no reverse action when the piston press-unit is rotated, thus it is possible to inject the precise amount of medicine to a patient with constant pressure.