A deformation mechanism for ridge-shaped kink structure (RSKS), a type of localized deformation, is studied with a discussion focusing on the kink deformation of a monocrystal with a single-slip system under a plane-strain condition. From a geometrical study of displacement continuity it is found that, to satisfy displacement continuity, the kink boundary formed by the deformation from an initially homogeneous structure must have symmetry. We propose a simple model of the RSKS deformation mode to accomplish plastic deformation from a compressive force parallel to the slip direction. First, important geometrical knowledge related to the RSKS formation mechanism is formulated analytically. Then, a simulation of a spring–mass model is performed to clarify the RSKS formation mechanism. The intensity of the angle-dependent force field is found to affect the deformation mode.