To provide a keyless entry system capable of automatically quickly adjusting an output level of an antenna.
This keyless entry system has a communication device 4 for transmitting a starting signal from the on-vehicle side antenna 11, and a portable remote controller 12 for sending back an identification signal for releasing a lock of a door lock device 3 by starting by the starting signal when a distance from the on-vehicle antenna 11 becomes within a predetermined distance. The communication device 4 has a controller 5 for storing output of the starting signal in a memory 9 when receiving the identification signal, by automatically changing the output of the starting signal, until the remote controller 12 arranged in a predetermined distance position sends back the identification signal to the on-vehicle side antenna 11 by starting by the starting signal, when setting the output of the starting signal in response to the remote controller 12.