To provide a device for preventing liquid-mixing of a tank lorry by which careless mistakes of drivers or operators can be prevented and mixing accidents of different kinds of oil or liquid can be securely prevented primarily, and the preventive system can be simply and easily realized secondarily.

An oil kind holder corresponding to a loading kind of oil of an unloading objective tank 3 is charged in a common advance space of the piston rods 36 of respective cylinders 31 corresponding to the tank 3. When the oil kinds holder is set, the working switch 20 is opened and compressed air is supplied to the opening/closing switch 14 of the bottom valve and respective cylinders 31. And only the piston rod 36 of the cylinder 31 corresponding to the loading kind of oil can advance through the confirmation hole of the oil kind holder. A sensor 34 detects the advancing movement of such a piston rod 36 and a voice-sounding means 35 informs of the oil kind name of the loading oil of the basis of detection of the sensor.