Electron energy loss spectroscopy (EELS) under reflection high-energy electron diffraction (RHEED) conditions has been measured and analyzed to investigate incident and exit angle dependence of plasmon excitation processes. As a specimen, clean Si(111)7×7 surface is used. Mean number of surface (n_s) and volume (n_ν) plasmon excitations has been obtained by Poisson analysis of the EELS spectra. Dependency of n_s on incident and exit angle of electron is slightly different from that expected from Lucas theory. It is considered that refraction effect of electrons near surfaces should be included for quantitative analysis. The n_ν value does not depend on incident and exit conditions within our experimental range, that indicates little dependence of effective depth for volume plasmon excitation under grazing incident conditions. From EELS spectra of Kikuchi lines and energy filtered RHEED patterns, it has been revealed that volume plasmon excitation enhancement only occurs on Kikuchi lines around a specular spot.