In the present paper, the electron transport equation has been applied to study the phenomena of cavity ionization. It has been shown that the effect of the cavity on the energy spectrum of electron inside it can be attributed to an equivalent electron source uniformly distributed in the cavity. A formula for the energy spectrum of the equivalent electron source is obtained. The deposited energy of the equivalent electron source in the cavity of a plate chamber has been calculated. As compared with the Spencer-Attix theory, the present theory is shown to be in better agreement with experiment for describing phenomena of the cavity ionization.