An injector for liquid medicine comprises a cylindrical body 1, first and second gasket 2, 3 contained in a lumen of cylindrical body 1. The cylindrical body 1 is provided with a fluid inlet 13 and a fluid outlet 14 in opposite sides of the cylindrical body 1 with respect to the longitudinal axis thereof. Second gasket 3 is spaced from the first gasket 2 to form a fluid chamber 4 between them. The first gasket 2 is adapted to be moved by sliding along the inner wall of the cylindrical body 1 from a first position to the distal end 11 of cylindrical body 1 via a second position. The fluid inlet 13 is opened at a position closer to the distal end 11 of the cylindrical body 1 than the outlet 14 is. The first gasket 2 is stopped at a second position where the fluid chamber 4 is communicated with the fluid outlet 14 but prevented from fluid-communication with the fluid inlet 13.