To provide a preventive device against mixing of liquid in a tank truck by which mixing accidents of oil or liquid can be easily and securely prevented and which can easily and manually cope with these accidents at troubles of the device.

In this preventive device 15 against mixing of liquid, a loading liquid is designated by a designation means of a loading liquid on loading. And an unloading chamber and an unloading liquid are designated by designation means of a chamber and a liquid on unloading. A control valve in a chamber is opened and a cylinder 19 is driven only when the loading liquid information of a chamber designated by the designation means of chambers coincides with the unloading liquid information to elastically displace an elastic hook 18 from the lock position C to unlock position D. Accordingly, the delivery valve V is changed over to 'open' and the liquid is unloaded by turning the handle 7 from the 'close' position A to the 'open' position B.