A circuit module and a measurement method enable accurate measurement of isolation characteristics of an isolator. The circuit module includes a processing circuit including SAW filters and a power amplifier and performs certain processing on an input signal. An isolator includes an input port, an output port and a ground port and outputs an input signal, which is input thereto via the input port and on which the processing circuit has performed the certain processing, as an output signal via the output port. A directional coupler distributes a signal transmitted between the processing circuit and the isolator. A measurement instrument inputs a test signal from the output port and measures a detection signal output from the directional coupler.