A locking device for locking a plunger rod of a syringe after use and preventing re-use of the syringe, wherein the syringe includes a barrel having an open proximal end and a flange provided adjacent said open proximal end, wherein the barrel is configured to receive the plunger rod through said open proximal end and a proximal end of the plunger rod is formed disc-shaped, said locking device comprising a mounting portion configured for mounting the locking device to the flange adjacent said proximal end by positive-fit, and a plunger rod locking portion for locking an axial position of the plunger rod after use of the syringe, wherein a communication channel is formed in the locking device for enabling insertion of the plunger rod into the open proximal end of the barrel via the plunger rod locking portion and the mounting portion, characterized in that a width of the communication channel is larger inside the plunger rod locking portion to form a chamber for receiving the disc-shaped proximal end of the plunger rod after use of the syringe, and the plunger rod locking portion comprises at least one locking nose radially protruding into the plunger rod locking portion and being configured to positively lock the axial position of the plunger rod by positive-fit engagement with the disc-shaped proximal end of the plunger rod, for locking the plunger rod at or near a foremost axial position of a distal end (51, 53) of the plunger rod in the barrel after use of the syringe, wherein the plunger rod locking portion comprises a circumferential side-wall and the at least one locking nose is each provided on an inner side of the circumferential side-wall, or is formed as a cage-like cylindrical member formed by two annular webs interconnected by axial connecting webs, wherein the at least one locking nose is each provided on an inner side of an annular web or of an axial connecting web. the syringe includes a barrel having an open proximal end and a flange provided adjacent said open proximal end, wherein the barrel is configured to receive the plunger rod through said open proximal end and a proximal end of the plunger rod is formed disc-shaped, said locking device comprising a mounting portion configured for mounting the locking device to the flange adjacent said proximal end by positive-fit, and a plunger rod locking portion for locking an axial position of the plunger rod after use of the syringe, wherein a communication channel is formed in the locking device for enabling insertion of the plunger rod into the open proximal end of the barrel via the plunger rod locking portion and the mounting portion, characterized in that a width of the communication channel is larger inside the plunger rod locking portion to form a chamber for receiving the disc-shaped proximal end of the plunger rod after use of the syringe, and the plunger rod locking portion comprises at least one locking nose radially protruding into the plunger rod locking portion and being configured to positively lock the axial position of the plunger rod by positive-fit engagement with the disc-shaped proximal end of the plunger rod, for locking the plunger rod at or near a foremost axial position of a distal end (51, 53) of the plunger rod in the barrel after use of the syringe, wherein the plunger rod locking portion comprises a circumferential side-wall and the at least one locking nose is each provided on an inner side of the circumferential side-wall, or is formed as a cage-like cylindrical member formed by two annular webs interconnected by axial connecting webs, wherein the at least one locking nose is each provided on an inner side of an annular web or of an axial connecting web.