The invention relates to an injection device, comprising an injection syringe (1) with a tubular syringe body, provided at a porximal end with a fixing collar (16), projecting radially outwards, an injection piston, arranged to slide within the body, a needle protection device (2), comprising a protector support (20) and a sliding needle protector (22). The device comprises a stop pin (25) for the piston stop (8), said pin (25) is mounted on the protector support (20) and defines, in the extension of the syringe body, a piston circulation passage of which the minimum section is larger than the section of the running part (10) of the piston and less than the section of a shoulder (15) of the piston.