PURPOSE: To provide a system for preventing mixing of oil at the time of unloading, which allows unloading oil only when the kind of unloading oil corresponds with the kind of oil in the underground tank, when oil is unloaded from a tank lorry in an oil station.

CONSTITUTION: A branch pipe 5 is provided on the way of a receiving pipe 1 to an underground tank 2 to divide the oil flow and a sample chamber 7 is provided in the downstream thereof for sampling. And a gas sensor 18 is equipped to judge the oil kind from the vapour of sample oil, in the sampling chamber 7. A control valve 4 is equipped in the upstream of branched point for the branch pipe 5 and a three-way valve 6 is equipped at the branch respectively, in the receiving pipe 1. The control valve and the three-way valve are controlled by a controller 21. Only when the oil kind judged by a gas sensor 18 is judged to corresponds with the oil kind stored in the underground tank 2 by the controller 21, the oil is unloaded into the underground tank 2.