The energy spectrum of the virtual photon field accompanied by a high energy charged particle which has been derived classically by Williams and Weizsäcker independently is derived here by the Feynman-Dyson method, and the equivalence between the Williams-Weizsäcker and the Feynman-Dyson methods is varified in a general way when the ehergy transferred from an incident charged particle is small. It is also qualitatively shown that the cross section derived by the Williams-Weizsäcker method gives a correct order of magnitude.