To provide a game controller that facilitates an operation, in the game controller operated by both hands.

A first game controller which can be used by both of a first game device and a second game device transmits operation data of the game controller by using a data format having a first data region in which a header is included, a second data region in which key data are included, and a third region in which data of an extension controller are included. A second game controller usable in the first game device uses the data format similarly having three data regions, however, the second data region is for only the securement of a region, and operation information is transmitted while being included in the third data region as a whole.