To improve the convenience of a user in mail communication.

A portable telephone set 101 transmits a SVGM (Scalable Vector Graphics Message) mail to transmission destination. The SVGM mail transmitted from the portable telephone set 101 is received by a mail server 40. The mail server 40 which has received the SVGM mail decides whether or not a transmission destination message data format decided corresponding to a terminal device at the transmission destination is an SVGM message format. When the result of decision is negative, the mail server 40 converts a section which can be adapted to the transmission destination message format in the SVGM message into the transmission destination message data format. Then, the mail server 40 transmits the mail including the conversion result to the terminal device at the transmission destination. The mail transmitted from the mail server 40 reaches the terminal device at the transmission destination.