The present invention provides a smart key system using Low Frequency (LF) antennas of a Tire Pressure Monitoring System (TPMS), in which a Central Processing Unit (CPU) installed inside a vehicle control locking modules of respective doors to be opened or locked in response to an open/close signal from a smart key configured to transmit/receive a wireless signal unique to the vehicle. A plurality of antennas are installed on the vehicle for receiving a low frequency control signal transmitted from the smart key. One or more of the antennas are the LF antennas used for the TPMS of the vehicle.