PURPOSE: To enable the coexisting of LAN and WAN lines by separating image data from data for video conference data, changing the compressing system of image data and compressing the image data again.

CONSTITUTION: When a LAN interface circuit 14 receives video conference data, a protocol conversion part 11 divides data into each of image, voice and control data. The image data is delivered to a compression elongation circuit 12 and the sound and and control data for which data conversions are unnecessitated is held. The circuit 12 elongates compressed image data, delivers the image data to a compression and expansion circuit 13 via a transmission bus 16, recompresses the image data for a WAN line and delivers the image data to the conversion part 11. The image data is coupled with the voice and control data held by performing protocol conversions by a coupling means, the image data is delivered to a WAN interface circuit 15 and the data is transmitted to a conference system terminal via the WAN line. Thus, the coexisting of the LAN and WAN lines becomes possible.