The Bowen fluorescence mechanism is investigated for the ionized gas cloud surrounding X-ray binaries. It is found that the helium-oxygen photon-conversion efficiency is about 0.5-0.8 at a Ly-alpha optical depth of 10 to the 7th in an expanding-cloud model or in a rotating-disk model. The helium-nitrogen photon-conversion efficiency through the oxygen-nitrogen resonance is about 0.1, which is close to the maximum conversion efficiency expected from the theory. The characteristic scale length of the cloud and the number density of He II are found. The derived cloud size and the observed radial-velocity shift with phase severely constrain the position of the cloud in the binary system. Two possibilities are considered for the gas cloud which is responsible for the optical emission lines: one is the gas of the stellar wind which is illuminated on one side by X-rays, and the other is the gas which is accumulated at the Langrangian triangular points.