To allow a sub-display device to be used not only for use application as a second monitor but also in presentation or discussion or the like in an electronic conference.

A portable sub-display device 100 is additionally equipped with a serial I/F processing part 101 in addition to an ADC part 110 and a video decoding processing part 111 while also being equipped with a pointing device 107 for a tablet or the like. Accordingly, in an electronic conference, it is possible to allow the portable sub-display device to have a function as a viewer of electronically-distributed materials together with a figure editing function useful during discussion, thereby expanding its application range besides application as a second monitor. The portable sub-display device 100 can be formed to be more inexpensive, lightweight, and compact than a notebook-size personal computer. Consequently, it allows the portable sub-display device to cope with cost problems and the problem of the lack in portability.