A square substrate has a pair of opposed major surfaces and peripheral end faces therebetween, wherein a tapered edge portion is disposed between the peripheral end face and each major surface to define an inner boundary with the major surface, and has a width of 0.2 1 mm from the peripheral end face. Both or either one of the major surfaces of the substrate has a flatness of up to 0.5 .mu.m in an outside region of the substrate that extends between a position spaced 3 mm inward from the peripheral end face and the inner boundary of the tapered edge portion.