A customer information database storing at least a unique customer identification number allocated to the customers by an official body and information of an account number, an amount outstanding, password, and the like which correspond to the customer identification number are stored. When the AGC voltage is within a predetermined voltage range, the overall gain of the AGC amplifier circuit is varied by the variable-gain amplifier; however, when the AGC voltage is outside the predetermined voltage range, the overall gain is kept constant by the fixed-gain amplifier. 7\xb0 relative to the C plane and a nitride semiconductor layer laid on top thereof.