An automatic locking system for vehicles includes an electronic control unit. If it is determined that an electronic key is not located in a communication area of an inside-cabin transmitter, the control unit determines whether or not the electronic key is located nearby. As a result, if the electronic key is determined as being located nearby, followed later by a determination that the electronic key is not located nearby, comparison history is stored in a memory. If it is determined that comparison history is stored in the memory and four entry-and-exit doors are closed, the four entry-and-exit doors are locked.