A terminal device includes: a session initiation request message generating section for generating a message for starting a session with an external device and in which capability information on the terminal device is described; a capability information analyzing section for analyzing capability information on the external device described in a response message transmitted from the external device; a communication mode determining section for determining a communication mode between the terminal device and the external device based on an analysis result by the capability information analyzing section and the capability information on the terminal device; and a control section for, based on the determined communication mode, controlling a data conversion device connected to the network such that data from the terminal device is converted into data receivable by the external device, and such that data from the external device is converted into data receivable by the terminal device.