An X-ray diffraction study was made on a new polymorphic phase of InSe obtained by heating a layered crystal of the 3R(γ) polytype under pressures of 40 and 50 kbar and by quenching to ambient conditions. An analysis of the diffraction intensity data collected on precession and Debye photographs gave a monoclinic structure (space group C12 h) with four molecules in a unit cell. Upon transformation, new, short interatomic distances characteristic of covalent bonding are formed between indium and selenium atoms that were originally located in the adjacent layers. The structure is regarded as a modified version of the nonlayered InS-type structure. In the real structure of the high-pressure phase, however, there is a small amount of disorder in the orientation of the InIn bonds. Discussion is given on the reconstructive, layer-nonlayer transformation of InSe.