To obtain a resin composition which can give a cured product excellent in chemical resistance, dielectric properties, heat resistance, and flame retardancy by mixing a polyphenylene ether resin with a 3,4-dihydro-3- substituted-1,3-benzoxazing compound and a phosphoric ester having a 3,4- dihydro-4-substituted-1,3-benzoxazine ring in a specified ratio.

This composition comprises (A) a polyphenylene ether resin, (B) a 3,4-dihydro-3-substituted-1,3-benzoxazine compound or a mixture thereof with a ring-opening polymer thereof, and (C) a phosphoric acid ester having a 3,4-dihydro-3-substituted-1,3-benzoxazine ring, desirablybeing a compound of the formula in such amounts that, on the basis of 100 pts.wt. total of components A and B, the amount of component A is 98-20 pts.wt., that of component B is 2-80 pts.wt., and that of component C is 1-50 pts.wt. In the formula, a+b+c=3; a is 1-3; b and c are each 0-2; R1 is phenyl, CH3, or cyclohexyl; and R2 to R6 are each H or an alkyl.