To obtain a coaxial line communication apparatus capable of improving communication quality in coaxial line communication while suppressing leakage of unnecessary radio waves from an antenna.

A modem 3A which is the coaxial line communication apparatus for performing communication by using a coaxial line 10 as a transmission line of a communication signal includes: a detection portion 25 for detecting whether power is fed through the coaxial line 10 or not; a determination portion 26 for determining whether a mixer or a booster 9 for attenuating a communication signal flowing out into an antenna 11 connected to the coaxial line 10 is arranged or not based on a detection result of the detection portion 25; and an adjustment portion 50 for adjusting a communication signal level of the communication signal based on a determination result of the determination portion 26.

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