A control portion decides whether received transmission data is destined for the processing board itself, based on a board ID included in a beginning header of the headers separated from the transmission data. If such is the case, a signal-processing portion processes an image signal separated from the transmission data or directly input, based on processing information (command and control parameter) included in the beginning header. A header generation portion generates a header to be combined with a processed image signal, based on the processing information included in the header. The header is combined with the image data to obtain transmission data. The processing board determines a transmission destination by itself.