To reduce the connection time of a communication line and to improve the transmission efficiency by using a moving image detection circuit to detect a moving image for each frame and sending multiplex data to the communication line only when a moving image is detected.

A motion detection circuit 103 compares moving image data 101 of this frame with moving image data of a preceding frame stored in a frame memory 114 to detect a motion with a small overlap error. Then a moving image detection circuit 115 detects it that a total sum of absolute values of moving positions for each frame exceeds a set value, image variable length data from a variable length data coding circuit 107 and audio variable length data from an audio coding circuit 198 are multiplexed by a multiplexer circuit 109 and the multiplexed data are sent to a communication line via a transmission circuit 110. Furthermore, when no motion is detected within a prescribed time, the moving image detection circuit 115 interrupts the transmission circuit 110. Thus, the connection time of the communication line is reduced and efficient transmission is attained.