The invention relates to a syringe gasket detachment preventing mechanism. The mechanism comprises a stopper fixed in a rear end portion of the syringe. By reducing a diameter of the stopper at an inserting portion of a rod portion of a plunger rod, the diameter is smaller than an inner diameter of the rear end portion to be inserted into the rear end portion. After the insertion, the diameter is enlarged so that a first concave portion of an outer circumference is engaged to a second convex portion of an inner surface of the rear end portion. Therefore, the stopper can be fixed in the rear end portion in an axially immovable manner. The stopper has a third convex portion on its inner surface. When the stopper is fixed, the third convex portion has an inner diameter smaller than an outer diameter of a gasket support flange of the rod portion.本發明之注射器之墊片脫落防止機構包含可固定於注射筒之後端部內之止擋件，且藉由使止擋件在柱塞桿之桿部之嵌入部縮小直徑，直徑會小於後端部之內徑，從而能夠插入至後端部，且於插入後直徑擴大而使外周之第1凹部嵌於後端部之內表面之第2凸部，藉此可固定於後端部內且無法軸向移動，止擋件於內表面具有第3凸部，該第3凸部於止擋件被固定之狀態下具有較桿部之墊片支持凸緣之外徑小之內徑。
