HAPS (high altitude platform station) is allowed to be used as a base station to provide IMT-2000 service. ITU-R has been requested to conduct studies, as a matter of urgency, on the feasibility of utilizing HAPS system to provide IMT-2000 service. One of the most important parts of the above feasibility studies is interference analysis, since HAPS system as a base station should not cause harmful interference to the cellular IMT-2000 system. On the sharing problems between two systems, the self interference from the adjacent cells in cellular system must be considered as well as the interference from HAPS system. In this paper, we estimated the interference to cellular mobile station from HAPS system by interference power, and finally the carrier to interference power ratio (C/I) with various system parameters, such as the number of HAPS users per cell and the multibeam cell radius of HAPS. With the estimation results, separation distance between a cellular system coverage and HAPS coverage for sharing between two systems can be determined.