A gasifying furnace gasifies solid fuel or liquid fuel. This method includes preparing a dummy substrate having a metal film formed on a surface thereof and a monitor substrate on which a cleaning evaluation is performed, and polishing the dummy substrate. As a result, signals from the remote control side can be received even while someone is listening to the radio. Completing the call by encoding voice signals using a common voice compression algorithm executed by the digital wireless telephones and by sending the encoded voice signals through a data network eliminates the drawbacks associated with the additional intermediate encoding which would have been required if the call had been completed using the circuit switched network.