PURPOSE: To grasp accurate data residence quantity in a buffer memory by using a variable length coding means so as to convert only a variable length code of a code word subject to variable length coding into a serial signal, storing it into a buffer memory and sending it to the transmission line.

CONSTITUTION: A variable length coding means 111 applies variable length coding to an inputted parallel signal and gives a code word and a code length subject to the variable length coding to a parallel serial conversion means 121 when the parallel serial conversion means 121 is indicated to be a reception enable state by a state indication signal. Moreover, the parallel serial conversion means 121 converts the signal into a serial signal of a code word in response to the code length and the serial signal is sent to the transmission line via a buffer memory 131. That is, only the effective data of the code word subject to variable length coding by the variable length coding means 11 is converted into a serial signal, stored in the buffer memory 131 and sent to the transmission line. Thus, the data resident quantity is accurately grasped.