In a transmitter device of a smart entry system for a vehicle, a transmitter antenna is provided between a variable power circuit and the ground, and is controlled by a switching circuit to transmit a searching radio wave. The transmission power of the antenna, that is, a range of reach of the searching radio wave is variably controlled by a drive output voltage applied to the antenna by a variable power circuit, which converts a battery voltage to the drive output voltage. Data to be transmitted in the searching radio wave is not used to modulate the drive output voltage but is used in an ON/OFF control of the switching circuit.