To obtain a high intensity, high definition and sophisticated electrooptic display device unit such as transmissive type LCD, a semi-transmissive type LCD, a reflective type LCD, a surface emitter type organic EL, or an underside emitter type organic EL, etc. which has high electron and positive hole mobility and low leak electric current qualities. A porous semiconductor layer (low porous Si layer 11a/high porous Si layer 11b/low porous Si layer 11c), a monocrystalline Si layer12a, and the Si O₂ layer 13a are formed on a monocrystalline Si substrate 10. The Si O₂ layer 13a of the peripheral circuit area is removed, leaving the Si O₂ layer 13a in the display area. The poly Si layer 14a is formed in the display area by semiconductor epitaxial growth, and a monocrystalline Si layer 12b is formed in the peripheral circuit area. Then, the display element section is formed in the poly Si layer 14 of the display area and the peripheral circuitry section is formed in the monocrystalline Si layer 12b of the peripheral circuit area. The assembly is divided into each ultra slim electrooptic display device unit after separation from the backing, and after the process of separating the Si substrate 10 from the porous Si layer 11b and by attaching the backing to the ultra slim electrooptic display element substrate after its separation. Each ultra slim electrooptic display device unit is divided after attaching the backing.