A method for laying planting soil on a rooftop flower bed of a building is provided to enable the roots of garden trees to become strong by supplying oxygen and carbon to the roots of the garden trees and delivering moisture to the planting soil. A retaining wall(10) is installed on the rooftop concrete slab of a building in constant height. A waterproof layer is installed on an inner side of the retaining wall. A waterproof protective layer(30) is installed on the inner side of the waterproof layer. A carbon-based humectant(40) is laid 30-40cm height from the bottom of the retaining wall. A plurality of cylindrical hollow bamboo tubes(50) are arranged to supply oxygen to the roots of garden trees. A nonwoven fabric mat(60) is positioned on both an outer side of the hollow bamboo tubes and an upper face of the carbon-based humectant. The nonwoven fabric mat is filled with planting soil(70). Landscape plants are planted in the plating soil. One end of a drainpipe(11) is located on the outer side of the retaini
