In a mobile radio communication system of a multi-layer cell structure for covering service areas in layers with cells of different sizes, an enquiry originating mobile terminal transmits enquiry information in an enquiry frame to the N-th layer exchange through the N-th layer base station. The exchange discriminates a kind of the enquiry frame at processing, selects at processing, a minimum cell for covering a distribution area or a minimum number of cells which can cover the distribution area from the state of geographical distribution of the enquiry destinations, fills the numbers of the base stations for forming these cells and the numbers of the exchanges for covering these base stations in the enquiry frame, and transmits this enquiry frame to the corresponding exchange. The exchange checks the numbers of the base stations of the enquiry frame and transmits the enquiry frame to the corresponding base stations. The base stations transmit the received enquiry frame directly to the mobile terminals of the destinations.