PURPOSE: To propose a liquid mixing preventing device for a tank lorry constituted in such a way as to correctly unload only the liquid to be unloaded so as to positively prevent the generation of oil mixing accident and liquid mixing accident of different kinds of liquid, and moreover excellent from the viewpoint of cost with simple constitution and easy to operate with high reliability.

CONSTITUTION: In this liquid mixing preventing device 8, each loaded liquid type detecting means 10 is charged with the liquid type key 9 of loaded liquid at the time of loading the liquid into each tank chamber 3 so as to obtain loaded liquid type information. At the unloading time, an unloaded liquid type detecting means 11 is charged with the liquid type key 9 of the liquid to be unloaded so as to obtain unloaded liquid type information, and the tank chamber 3 to be unloaded is instructed by a tank chamber instructing means 12. The loaded liquid type information and unloaded liquid type information of this tank chamber 3 are compared by a collating means 13, and in the case of both information coinciding with each other, the lock of a locking device 14 for the bottom valve 4 of the tank chamber 3 is released to enable unloading.