A kind of structural phase transition is considered in which intrinsic lattice instability is incommensurate but a commensurate structure locks in eventually bypassing a possible intermediate phase of incommensurately modulated structure. The Verwey transition in magnetite (Fe3O4, Tv =120 K) is taken as an example of this kind of transition explaining puzzling features of the transition such as the incommensurate diffuse scattering above Tv which is not apparent related to the structure below Tv and the appearance of secondary lattice distortion characterized by a wave vector qX= (0,0,1) in addition to the primary lattice distrotion with qΔ= (0,0,1/2).