fig1 illustrates protection effects 706 when redundancy code strength 704 and retransmission upper limit count 705 are adopted for each of real time streams 701 702 703 in the present embodiment document image 703 is very important information for the user to understand contents of a conference therefore in fig3 protection strength of the real time stream of document image 703 is set to strongest u201c 10 u201d a delay of 200 ms is allowed for the real time stream of document image 703 for the real time stream of speaker sound 702 in the present embodiment as shown in fig1 the redundancy code strength of the real time stream of document image 703 is set to 50 according to the information and the retransmission upper limit count is set to 2 as a result the real time stream of document image 703 is transmitted completely undisturbed in the present embodiment speaker image 702 is not so important information to the user however speaker image 702 is information that needs to be transmitted in synchronization with speaker sound 701 therefore in fig3 the protection strength of the real time stream of speaker image 702 is set to u201c 2 u201d which is lower than that of document image 703 the real time stream of speaker image 702 is allowed to have a delay of 10 ms with respect to the real time stream of speaker sound 701 in the present embodiment as shown in fig1 the redundancy code strength of the real.