In an in-building CATV system in which an upward signal is frequency-converted to an in-building upward signal using an up-converter at a terminal side and the frequency-converted in-building upward signal is outputted to an external system after frequency-converting the in-building upward signal to an upward signal for outputting using a down-converter, a high frequency signal for frequency conversion leaking out from the up-converter is occasionally transmitted to the down-converter. Then, a trap circuit for removing the high frequency signal is provided on an in-building upward signal path in each converter. And an equalizer is provided on an upward signal path in each converter to compensate a transmission loss in the frequency band of the in-building upward signal occurring when the in-building upward signal is passed through the trap circuit.