A mail server and mail terminals are connected through a radio communication network. The mail server makes viral infection decision of if e-mails are infected with computer virus without burdening the mail terminals having poor processing power. The mail terminals are constructed to serve as small-sized mail terminals. When an e-mail arrives at the mail server through the Internet, the mail server decides if the e-mail is infected with virus. The result of the viral infection decision is added as the whole or a part of inspection information within the header of the e-mail. The corresponding mail terminal can detect presence or absence of viral infection on the basis of the inspection information. When the e-mail is infected with virus, the mail terminal can inform a user of the contaminated e-mail by making tone of a buzzer or vibrator different from a normal e-mail.