The Military Airbase Law has designated imaginary surface, restricting the height of structure above the imaginary surface. The purpose of establishment of imaginary surface is to promote safety of aircraft operation. However, given the fact that the land is limited, the Military Airbase Law does not set up outer horizontal surface such that the reduced imaginary surface would undermine the safety of aircraft operations with respect to IFR (Instrument Flight Rules) procedures. The fact that some areas which IFR procedures require are not designated as imaginary surface results in no imposition of height restriction on such area. With regard to aviation safety, this article deals with issue of height restriction on area which is beyond the imaginary surface area. This article also examines the establishment of the imaginary surface in the Military Airbase Law, and conducts case study of imaginary surface systems in other country. Based upon this discussion, the current problems posed in the Military Airbase Law will be discussed. To resolve these problems, it is necessary to establish height restriction zone beyond the current imaginary surface area. The provisions of ICAO, FAA and other counties in relation with this issue are referred as well. As a result, many different proposals are suggested, and it is concluded as the most effective proposal that, in certain zone beyond the imaginary surface area, an aeronautic study should be required for the purpose of ensuring aviation safety before a permit of construction higher than prescribed height is issued.