PURPOSE: To transmit a STM signal having even any frame structure through an ATM network by installing a pointer in a cell information field.

CONSTITUTION: A STM transmission signal is treated for STM signal frame synchronism detection by the existing network interface section 1 to convert it to transmission data and a transmission frame synchronous signal that indicates the leading position of frame. These are sent to an assembly section 2 and are cellulated therein. The transmission cell, after it is treated for cell multi-conversion based on the format at a user network interface section 4, is sent as an ATM transmission signal to the ATM network. On the other hand, the ATM receiving signal, by treating it for cell separation conversion based on the format at the interface section 4, obtains a receiving cell, and outputs the receiving cell to a cell disassembly section 3. Pointer is interpreted, receiving data that detected the leading position of frame of the STM signal in the cell and a receiving frame synchronous signal, after it is converted to a signal format for STM signal at the interface section 1, is sent out as a STM receiving signal.