Electron microscope observation showed a detailed structure of microdomains of the Dauphiné twin appearing in the incommensurate phase between the α and β phases. The triangular microdomains form a hexagonal domain lattice in orientations slightly deviating from the {10.0} planes by angles ±φ, which gives rise to a `macrodomain' structure. From the contrast analysis of the domain boundary, a relative lattice displacement parallel to the boundary is expected between the neighbouring domains, and a model was proposed. A dynamic process of the α-β phase transition was observed using a TV camera system attached to the electron microscope.