This paper describes, based on generation mechanism of conductive noise, that the real conductive noise on AC-mains can't be measured by LISN and 50 Ω-input impedance instrument specified by regulations such as CISPR. Second, it is pointed out that one of the causes of poor reproducibility in radiated emission measurement is the difference among line impedances of AC-mains. Finally, it is insisted that the apparatus such as LISN is necessary for stable measurement of radiated emission, and what improvement on LISN characteristics for higher frequency range should be done is introduced.