PURPOSE: To obtain SMC without variance in its quality, by mixing a liquid thermosetting resin with a thickener coated with a substance which can be dissolved in or released by the resin, impregnating a reinforcing fiber with the obtained resin mixture and maturing this impregnated composition.

CONSTITUTION: A liquid thermosetting resin (e.g., styrene-containing unsaturated polyester resin) is mixed with a thickener (e.g., magnesium oxide) coated with a substance (e.g., zinc stearate) which can be dissolved in or released by the resin. The purpose SMC is obtained by impregnating a reinforcement fiber with the obtained resin composition and maturing this impregnated composition. In this way, a resin composition which can be controlled in thickening in the early stage after mixing the thermosetting resin with the thickener because it is coated with the substance and so has a long pot life can be obtained. This resin composition can give SMC of a sufficiently high viscosity because the coating substance can be gradually released from the thickener during curing.