To surely operate a safety device without fail in closing a filler cap.

This injector has an injection port 23 and the filler cap 1, the filler cap has an engagement part 4, the engagement part 4 and the injection port 23 are provided with complementary engagement elements 7, 8, 25 and 26 for forming, for example, a screwed joint or a bayonet joint, and these engagement elements are movable from a non-engagement initial end position to a terminal position through an engagement stroke, or on the contrary from the terminal position to the non-engagement initial end position by the movement of the filler cap 1 to the injection port 23. The safety devices 13 and 14 for preventing the filler cap 1 from being automatically removed, are mounted to increase the moving resistance within a specific moving range of the filler cap 1. The safety devices 13 and 14 are mounted to be effectively operated at positions separated from the terminal position by a predetermined distance.