There is provided an inter-main-apparatus interface device which, even if a first-standard main apparatus and a second-standard main apparatus are gathered in a main apparatus system, enables both of main apparatuses to function normally. The device comprises a control data interface provided between the first-standard main apparatus and the second-standard main apparatus for operating a controller included in the second-standard main apparatus on the basis of control data supplied from the first-standard main apparatus, and a content data interface provided between the first-standard main apparatus and the second-standard main apparatus for enabling the second-standard main apparatus, as controlled by the controller operating on the basis of the control data, to relay by multiplexing and demultiplexing, content data sent and received by a base station connected to the first-standard main apparatus and content data sent and received by a base station connected to the second-standard main apparatus.