To form a base course of a parking lot, which enables fixation of parking equipment, which hardly causes the growth of weeds, which can prevent the occurrence of a flood in an urban area by allowing rainwater permeation by high water permeability, and which is useful for suppression of a heat island phenomenon because moisture is evaporated from the base course by high water retentivity so as to lower air temperature.

A structure of an outdoor parking lot comprises an upper layer 4 for forming a rolled and paved road surface of the open parking lot, and a rolled lower layer 3 below the upper layer 4. The upper layer 4 contains 50-90 wt.% of porous granular coal ash with a particle diameter of 1.0-10 mm; the lower layer contains 70-95 wt.% of powdery coal ash with a particle diameter of 0.0001-0.9 mm; the upper layer 4 is superior in water permeability to the lower layer 3; and the lower layer 3 is superior in waterproof property to the upper layer 4.