PURPOSE: To unlock a door by calling telephone system in an automobile from the outside of the automobile even if the door key is left in the automobile.

CONSTITUTION: If the door is locked while the door key is left in the inside of the automobile, at first an outside cabine switch 7 is thrown to supply power to the automobile telephone set. The mounted telephone set is called by a number button 14 from a public telephone set 11 and a prescribed pass word is inputted from the button 14. A control circuit 3 in the automobile collates the received pass word with the stored pass word and gives an unlock signal to an unlock control circuit 5 when they are coincident to unlock the door. Through the constitution above, even if the door key is left in the automobile, the door is unlocked easily.