In a communication system for communicating a plurality of frequency signals between a first apparatus 1 and a second apparatus 2 via a common cable 3, a level loss component of each of the frequency signals in the cable is effectively corrected. Both transmission-sided reference frequency signal level detecting means 41 and 43 of the first apparatus, and reception-sided reference frequency signal level detecting means 71 and 73 of the second apparatus detect a level of a frequency signal which constitutes a reference among frequency signals which are transmitted from the first apparatus to the second apparatus via the cable. In the first apparatus, out-of-reference frequency signal level control means 44 to 51, and 32 to 35 control levels of the frequency signals other than the frequency signal which constitutes the reference and is communicated between the first apparatus and the second apparatus via the cable based upon a compared result between results detected in the first apparatus and results detected in the second apparatus.