PURPOSE: To measure a level of a CATV transmission line by outputting a part for a synchronous compression period as a regular level.

CONSTITUTION: A scramble timing pulse generating circuit 12 receives a signal separated by a synchronous separator section 11 and generates a scramble timing pulse based on a signal regulating a compression period. A video IF modulation section 13 compresses a video synchronization part of a video IF level in the generated scramble timing pulse. On the other hand, an audio IF modulation section 14 superimposes a timing pulse for demodulation on an audio signal at a terminal equipment. After a video IF signal from the modulation section 13 and an audio IF signal from the modulation section 14 are mixed by a mixer 15, the mixed signal is frequency-converted into a channel of a CATV at a frequency conversion section 16 and the result is sent to a transmission line. In this case, the video level on the transmission line is measured by outputting part of the vertical period at a regular level.