PURPOSE: To monitor a cell traffic pattern of each signal source after multiplexing and regulate the pattern by arranging cells from the signal source at an equal interval on a multiplex transmission line and allocating assigned cell slots at an equal interval again.

CONSTITUTION: A cell multiplexer multiplexes cells coming from plural signal sources and sends the result to a multiplex transmission line 40. A cell slot assignment circuit 3 is provided, which sets a speed sending a prescribed number of cells for each multiplex period to a basic speed and sets the speed being an integral number of multiple of the basic speed to a cell transmission speed of each signal source to allocate the slot according to a predetermined rule. A memory 4 stores the assignment of the cell slot and a buffer memory 1 and a selector 2 select and multiplex the cells from plural signal sources according to the storage content of the memory 4. The memory 1 stores cells coming from the signal source via a transmission line 20. The selector 2 sends cells of the memory 1 designated by a command from the memory 4 to the multiplex transmission line 40. When no cell exists in the designated memory 1, an idle cell is sent.