To provide a feeder length measuring instrument which is capable of easily measuring the length of a feeder used for connecting a communication circuit and an antenna in a radio communication apparatus and further can be accomplished at a low cost.

The measuring instrument comprises a Colpitts oscillation circuit 50 utilizing as a coil of a resonator a feeder (coaxial cable 3) of an antenna 4 connected through a diplexer 7 and a connecting terminal 6 of a communication apparatus. An oscillation frequency of the Colpitts oscillation circuit 50 is varied in accordance with a length of the coaxial cable and since the frequency becomes lower as the coaxial cable becomes longer, an oscillation signal from the Colpitts oscillation circuit 50 is input to a frequency/voltage conversion circuit constituted of an integration circuit 60 and a rectifier circuit 70, the frequency of the oscillation signal is converted into a DC voltage signal, and the DC voltage signal is output as a signal representing the length of the feeder.