Second and third ports of a network analyzer are individually connected via cables to predetermined connection points on a differential transmission circuit on an object to be measured. A differential cable is connected to the differential transmission circuit. An antenna for receiving an electromagnetic wave radiated from the differential cable is connected to a first port of the network analyzer via a first cable. The network analyzer measures a three-port S parameter of the first, second, and third ports and calculates common-mode and normal-mode components of noise radiated from the differential cable. As a result, the source of noise in an electronic apparatus can be determined, and common-mode noise and normal-mode noise can be separately measured.