A data transmission system including a plurality of terminal devices that are connected to a communication network and that transmit or receive content data among the terminal devices. At least one of the terminal devices that receives the content data includes a code amount change requesting unit that, based on an output time interval of an output signal corresponding to predetermined content data, transmits a code amount reduction request to request reduction of a code amount of the predetermined content data while designating a transmission source of the content data as a destination; and a code amount changing unit that, in response to a code amount reduction request transmitted from another terminal device, reduces a code amount of corresponding content data.