This study measured the indoor radon concentrations at 23 administrative office buildings in Gyeongju, Korea, which consists of 23 administrative districts. Using the Korean geological information system, the type of bedrock under the administrative office buildings was identified and classified in 3 major types: granite, sedimentary rock, and sedimentary rock-based fault. The changes in the indoor concentrations at the 23 administrative office buildings were analyzed according to the type of bedrock. As a result, the radon concentration in the areas with the granite bedrock was generally higher than that in the region of two other types of bedrock. In addition, the radon concentration was evaluated according to surface area and construction timing of the building. The indoor radon concentration generally increased with decreasing surface area of the building, particularly in granite distributed areas. For a building aged more than 15 years, the radon concentration in the building in the granite area was much higher. For the building aged 1 or 2 years, the radon concentration was high regardless of the type of the bedrock due to radon emanation from the building material, such as concrete.