Even in these days, almost small airports and heliports have no equipment for aircraft surveillance. Many air traffic operation and airport operation service providers then request to develop the equipment for monitoring airspace to improve their service performance. We then think it is important to make the most effective use of conventional Secondary Surveillance Radar (SSR) system currently widely used, and we propose a Passive SSR (PSSR) to meet the request. The PSSR is the slave system for a conventional SSR of the mode-A/C system. The PSSR is a very compact and potable system consisting an omnidirectional antenna to receive the SSR replies from aircraft, an antenna to receive the SSR interrogations from a master SSR system, and a digital signal processor such as a personal computer for identifying aircraft and calculating the position of the aircraft. The PSSR installed within the coverage of the conventional SSR can provide SSR mode-A/C information without any radio transmission as interrogation. An Airspace Surveillance Network will be able to be established economically by utilizing PSSRs and InterNet.