For investigating the interaction of He and Ne in the field adsorption and ionization process above a single tungsten atom, field ion signals have been directly measured as a function of time using a micro-probe hole field ion microscope combined with a pulse counting analysis. Observed ion signals clearly show the transitions between a field adsorbed He state and a field adsorbed Ne state, and enable the determination of average adsorption time of He and Ne. Simple probabilistic model of adsorption and exchange of atoms at field adsorption site taking account of gas densities close to the surface well reproduced the observed behavior of average adsorption times. The results indicate the pronounced increase of Ne gas density near the sample surface. Particularly in the case of an initial mixed ratio of less than 0.5% of Ne for He, the increment reaches more than a hundred times.