A lock system which prevents the mixing of different kinds of oil at a gasoline tank comprises: locks for different kinds of oils provided on each of couplings constituting oil filler ports of underground tanks of the gasoline stand and each of tank base valves of a tank truck, and keys separately provided for unlocking the locks for the different kinds of oil, wherein only the lock on the couplings of the oil filler ports which accepts a selected key which unlocks the corresponding tank base valve of the tank truck can be unlocked. When one of the tank base valves is opened and an oil filler hose is connected to the oil filler port of one of the underground tanks, only the oil filler port of the underground tank containing the same kind of oil as that in the corresponding tank of the tank truck can be opened, thereby ensuring that the same kind of oil is reliably poured into the underground tank from the tank truck. The keys are stored in a key-storing device which holds the keys separately for the different kinds of oil, and is constructed such that only one key for one kind of oil can be removed at a time. It is thereby possible to provide a lock system preventing the mixing of different kinds of oil at a gasoline stand, which can reliably prevent the mixing of different kinds of oil with simple devices and members.