compression and expansion circuit 7 607 and when a decision has been made that there is no remaining voice data the data transmission is terminated immediately 608 when a decision has been made that there is remaining voice data yes in 607 the cpu 44 resets the dma controller 51 through the high order cpu bus 33 the voice data stored in the data temporary storage circuit 8a is sequentially transmitted while the dma controller 51 is being reset therefore voice data which is being transmitted is not interrupted next the cpu 44 starts dma transfer to the local processor 4 through the high order cpu bus 33 604 similarly dma transfer is carried out until the number of dma transfer bytes reaches a set value when the dma transfer has been terminated a decision is made whether there is remaining voice data or not 607 if there is remaining data the cpu 44 starts dma transfer to the local processor 4 and this operation is repeated if there is no remaining voice data the data transfer is terminated when a decision has been made that the mode is the compression receiving the cpu 44 receives a report to this extent from the local processor 4 through the high order cpu bus 33 upon receiving the report of the receiving of the voice data the cpu 44 sets a number of bytes of the voice data to be transmitted to the dma controller 51 of the local processor 4 through the high order cpu bus 33 in one dma transfer.