The invention concerns the field of pre-filled and disposable needleless syringes, operating with a gas generator and used for intradermal, subcutaneous and intramuscular injections of a liquid active principle for therapeutic purposes in human and veterinary medicine. Said injection device (1) is mainly characterized in that it has a free pressuring space (26) around the reservoir (20) of the active principle (17) so that the gases produced by the gas generator (4) instantly penetrate said pressurizing space (26), compressing said reservoir (20) just before causing the plunger to move (18) to expel the liquid active principle (17). During said displacement, the reservoir (20) is subjected to a state of equal pressure.
