A mold assembly for molding a thermoplastic resin, which comprises; (a) a first mold member and a second mold member for manufacturing a molded article of a thermoplastic resin, (b) an insert block provided in the first mold member, said insert block constituting part of a cavity, having a thickness of 0.1 mm to 10 mm and being formed of a material having a thermal conductivity of 2.times.10.sup.-2 cal/cm.multidot.sec.multidot..degree.C. or less, and (c) a molten thermoplastic resin introduction portion provided in the second mold member, wherein an insert block covering portion is formed in the second mold member, and when the first mold member and the second mold member are clamped to each other, (A) a clearance between the insert block and the insert block covering portion is equal to, or less than, 0.03 mm, and (B) an amount of overlapping of the insert block and the insert block covering portion is equal to, or greater than, 0.5 mm.