A vehicle passive entry system having a portable unit carried by a driver, the portable unit comprising an operation key for locking or unlocking a filler cap on a fuel tank which is adapted to move between a protruding position where a locking or unlocking operation can be performed on the filler cap and a fully accommodated position where the locking or unlocking operation cannot be performed and a Hall element for detecting that the operation key is located at the protruding position. When the Hall element detects that the operation key is located at the protruding position, a predetermined radio wave is transmitted from the portable unit 1 to the vehicle side, so that an engine is stopped.