This paper describes constrained sintering of substrate with sandwich structure, which was laminated with inner-constraining alumina layer between glass-alumina mixed layers. The inner-constraining alumina layer doesn't shrink at the sintering temperature of the glass-alumina mixed layer, and therefore, the glass-alumina mixed layer doesn't shrink in X-Y direction. The inner-constraining alumina layer is, densificated by penetration of molten glass from glass-alumina mixed layers. The bending strength of this substrate is increased.