We have studied the decay process of a silicon mound on the Si(100)-(2x1) surface at 500°C. The mound has been fabricated by an STM tip. The shape of the mound is a quadrangular pyramid with facets of regular array of steps that have double layer height. For the all step, the dimer rows are perpendicular to the step edges, so called the D@sub B@ step. Just after fabrication, the pyramid begins to decay layer-by-layer. During the decay, area of the bottom single layer of the pyramid is grown toward the dimer row direction. The layer width does not change toward perpendicular to the dimer rows. The area of the topmost layer of the pyramid decreases toward perpendicular to the dimer rows, because the both S@sub B@ step edges are pinned by the step edges of the lower ones. Therefore the D@sub B@ step edges remain at the each step edge, and move scarcely. But only the S@sub B@ steps at the bottom layer move due to attachment and detachment of atoms which are detached from the upper layers. The decay process of a single layer mound on the Si(100) surface is very different from that of the topmost layer of the pyramid. The S@sub B@ steps fluctuate greatly. Roughness of the S@sub B@ steps changes periodically. The aspect ratio of the mound oscillates between about 2.5 and 1.5. The decay rate of a single layer mound varies with each mound. The rate is affected by the defect of the surface. The increase of the defect ratio causes the decrease of the decay rate.