This study distinguished between subjective and objective knowledge of scientific technology whose influence on health is yet to be defined, and explored the effect of each type of knowledge on risk perception. A web-based survey among the Japanese population (N＝1,110) was conducted. It assessed the subjective and objective knowledge of electromagnetic field (EMF), interest in EMF, risk perception, and trust on the related organizations. The results indicated that respondents’ objective knowledge about EMF was generally poor. Their subjective knowledge and objective knowledge were significantly correlated, but the strength of correlation was moderate. Multiple regression analysis yielded significant subjective×objective knowledge interaction on risk perception. While objective knowledge consistently attenuated risk perception, subjective knowledge boosted risk perception only when respondents’ objective knowledge level was low. The possibility of the potential influence of reputation of EMF on the boosted subjective knowledge about EMF was discussed.