A keyless entry device includes a plurality of antennas which receive high frequency signals, a high frequency receiver which reproduces data from the high frequency signals received by the plurality of antennas, an intermittent controller which changes over an operation of the high frequency receiver between a continuous operation and an intermittent operation in an interlocking manner with turning on or off of an ignition key of an automobile, and a data processor which outputs data as control signals, wherein when the ignition key is turned on, the keyless entry device is operated as the tire pressure monitoring device and when the ignition key is turned off, the keyless entry device is operated as the keyless entry device.