Abstract: In a cell assembling device, an ATM cell assemblage decomposes input signals channel by channel and outputs channel-by-channel signals. At the same time, the cell assemblage generates a write control signal. Further, when one cell of data is written to a buffer, the cell assemblage generates a condition report signal for informing an arbitrating circuit of the channel of the cell. In response, the arbitrating circuit determines a channel to read out on the basis of the amounts of cell data stored in the buffer, and then outputs a read control signal meant for the above channel. On receiving the write control signal, the buffer writes the channel-by-channel signal therein. In response to the read control signal, the buffer outputs the signal of the channel designated by the signal and feeds it to a cell transmission circuit. The transmission circuit adds a cell header to the data output from the buffer to thereby form a cell to be sent.