Provided is a system and method for updating a plurality of the same files consistently. The dielectric housing has a first body. The metal preprocessed layer may reduce oxidation of the lower electrode due to oxygen supplied during formation of the dielectric layer. The period for holding the voltage is different between the first and second drivings. The echo canceller 7 generates a pseudo echo signal based on the low-frequency suppressed received signal passed through the high-pass filter 11, and eliminates the acoustic echo to be inputted to the microphone 4 from the speaker 3 by subtracting the pseudo echo signal from the digital signal converted by the AD converter 6. In a method of this type, deposits are prevented in a simple and economic manner by metering an additive into the steam, which additive adheres to the steam-side surface of the steam system and has the effect of repelling moisture and contaminants.