PURPOSE: To prevent oil of a different kind from that reserved in a reservoir tank at a gas station from being mixed from a tank truck by mistake.

CONSTITUTION: When a magnetic card X is inserted into each of boxes 5 above respective sections 2b, a switch valve in each section 2b closes an air pipe P2. Thus a bottom valve 3 cannot be opened even if a bottom valve control switch 4 is operated. When the magnetic card X is removed from the box 5 of the section 2b into which oil is to be unloaded at a gas station, the switch valve in the box 5 opens the air pipe P2. Thus only the bottom valve 3 of the section 2b into which oil is to be unloaded can be opened by the bottom valve control switch 4.