To unnecessitate an instruction from a remote transmitter-receiver, to be economical without any increase in a circuit scale resulting from an ATM cell formation and also to control packet data transmission quantity with little errors.

An encoder 2 converts the packet data into continuous code word strings suitable for optical fiber transmission. A code work counter 22 and an idle counter 23 arranged on the post stage of the encoder 2 accumulatively count the number of all code words and the number of code words of an idle set. Meanwhile, a controller 24 outputs an inhibiting signal 1c to a data packet generation source 1 and suppresses packet generation when the value of an up-down counter 25 performing up count and down count of the both count values in each prescribed value surpasses a prescribed threshold. Then, data packet transmission quantity can be adjusted without receiving an inhibiting signal from a remote opposite transmitter-receiver or without performing ATM cell assembling and SDH framing.