Recently, the indoor air pollution by microbes such as fungi and mites have become a concern as important research topic on indoor air quality. Fungal growth is significantly affected by humidity. In this study, we examined the influence of relative humidity on the surface of building materials and the water content of building materials on the fungal growth rate by measuring the mycelium length of fungi in the fungal detector placed on the surface of building materials. As a result, even if the relative humidity on the surface of building materials is identical, the more water content of building materials is, the more fungi grow faster. It was suggested that fungal growth rate depends on not only the relative humidity on the surface of building materials but also the water content of building materials.