Abstract : The physics of the Galactic diffuse emission is an important topic in GeV gamma-ray astronomy as it allows the study of charged cosmic rays in our Galaxy. As there are only few measurements above 10 GeV, we have searched for these diffuse emission in the OFF-source data of the CELESTE experiment, which is the first atmospheric Cherenkov detector with an energy threshold below 100 GeV. The production and study of a large set of Monte Carlo simulations showed the simulation/data compatibility for gamma rays and allowed estimations of the CELESTE sensitivity of diffuse gamma-ray radiation. With the intent of detecting Galactic diffuse gamma-rays mostly located at low Galactic latitudes, we performed the analysis of some CELESTE OFF source data, comparing data taken near the Galactic plane with data taken away from the Galactic plane. Data used in this study was selected with criteria based on atmospheric and acquisition stability. A first step in this analysis was to verify that there is no significant systematic effect in the OFF / OFF pair association of high Galactic latitudes data taken at different dates but at the sames local coordinates in the sky. A second step was to perform OFF-OFF analysis with other OFF acquisitions taken at different dates to search for an event excess from the data acquisitions the nearest to the Galactic equator. Since models and extrapolations from fluxes measured at lower energies predict a rather low signal, we used an analysis method based on a composed discriminant variable---based on the geometric and timing properties of the Cherenkov wavefront---to improve the sensitivity of the instrument to discriminate between signal and background events. With 108 min of data in the Galactic anticentre region, an upper limit with a 95 % C.L. was provided at 100 GeV, giving a limited integral flux of the Galactic gamma-ray diffuse emission: phi (E=100 GeV) = 6.6 x 10^-3 m^-2 s^-1 sr^-1.