A synthetic lamellar magnesium phyllosilicate has as the main component a composition represented by the following formula:Mg.sub.3 Si.sub.4 O.sub.10 (OH).sub.2.nH.sub.2 O (1)wherein n is a number of up to 5, and has X-ray diffraction peaks at spacings of 4.5-4.6 .ANG., 2.5-2.6 .ANG. and 1.5-1.6 .ANG., wherein the lamination asymmetry index (Is) defined by the following formula:Is=tan .theta..sub.2 /tan .theta..sub.1 (2)wherein.theta..sub.1 stands for the angle formed by the peak perpendicular and the peak tangent on the small diffraction angle side in said X-ray diffraction peak at a spacing of 4.5-4.6 .ANG., and.theta..sub.2 stands for the angle formed by the peak perpendicular and the peak tangent on the large diffraction angle side in said X-ray diffraction peak,is at least 3.0, the BET specific surface area is at least 300 m.sup.2 /g and the Methylene Blue decolorizing power (JIS K-1470) is at least 100 ml/g. This synthetic lamellar magnesium phyllosilicate is prepared by a process comprising subjecting active silicic acid or active aluminosilicic acid and an oxide or hydroxide of magnesium or a compound capable of forming said oxide or hydroxide under reaction conditions to a hydrothermal treatment.