Disclosed is a one-time use safety injector capable of easily removing a remaining solution in the administration of an injection solution and easily being detached and discarded after use. The automatic safety injector according to the present invention includes a needle flange 31 having a needle 30 mounted in the front thereof and a cone inner space 32 and an edge portion 33 formed in the rear thereof, a coupling ring 50 being inserted into a circumferential edge of the edge portion 33; an adapter 60 including a hanging projection 61, a cone protrusion 62, coupling ring insertion grooves 63, a spherical flange 64 and a gasket 70, the gasket 70 being inserted from the front circumferential edge up to the hanging projection 61, the cone protrusion 62 being projected and installed in its head portion, the coupling ring insertion grooves 63 being formed around the cone protrusion 62, and the spherical flange 64 being formed in the front of the adapter 60; a piston 40 including elastic wings 41,41′ and a spherical flange 42, the elastic wings 41,41′ being installed in both sides of the head portion of the piston 40, and the spherical flange 42 being projected and installed inside the piston 40; and an injector cylinder 20 having an injection solution inlet 21 formed in the front thereof and an adapter hanging projection 22 formed in the rear thereof.