The mechanism of décollement formation was investigated through a particle-based simulation model assuming homogeneity (e.g. no weak layer or pore fluid). A décollement-like structure appeared as a spontaneously localized shear deformation near the bottom of the sediment when the thickness of the sediment was sufficient to balance the gravitational force and tectonic loading. In contrast, no such décollement-like structure was formed when the sediment was too thin; in this case, the entire prism was deformed because of plate motion. These results are consistent with various observations in real subduction zones. A precise analysis of the stress state evolution during accretion reveals that the formation of a décollement-like structure is controlled by the spatio-temporal distribution of isotropic compression states.