A remote key entry system has a key unit and an answerback sound producing device. The key unit transmits a radio signal for remote-controlled locking and unlocking of a vehicle. The answerback sound producing device receives the key signal to indicate completion of correct locking or unlocking of the vehicle. The remote key entry system includes an environmental data sensing devices for obtaining data of an environment around the vehicle for control of the answerback sound. The system also has an answerback sound control device for controlling at least one of sound volume and tone of the answerback sound based on the environment data. The environmental data sensing device is an illumination sensing device for detecting environmental illuminance around the vehicle. The answerback sound control device varies the sound volume proportional to the illuminance. The environmental data sensing device may be a location calculating device for knowing location of the vehicle, and the answerback sound control device varies the sound volume based on the location of the vehicle.