We propose a fabrication method of three-dimensional silicon slopes using RIE-lag. RIE-lag is a lag of an etching rate depending on square openings area of a mask. We measured relationship between area of square openings and etched depths. We confirmed that etched depths were defined as a function of the square openings. With this relationship, we designed a mask with various sizes of the squares for slope structures. Square openings of various sizes were patterned using EB lithography. Silicon was etched vertically with ICP-RIE (Inductive Coupled Plasma - Reactive Ion Etching). By RIE-lag, trenches with multiple depths depending on the area of the square openings were formed. Silicon surface was smoothed by SF6 isotropic dry etching. As a result, by the combination of ICP-RIE anisotropic etching RIE-lag and SF6 isotropic etching, we fabricated 57° silicon slopes of surface roughness 10 nm in plane and 35 nm in slope surface.