The thickness of a fiber-reinforced resin sheet having uniformly distributed and substantially unidirectional fibers is reduced by stretching a fiber-reinforced resin sheet starting material in intimate contact with a thermoplastic sheet having a tensile strength larger than the fiber-reinforced resin sheet. This is accomplished by heating the fiber-reinforced resin sheet at a temperature at which the resin in the fiber-reinforced resin becomes sticky and subjecting the sheets to a uniform compressive force to adhere the fiber-reinforced sheet to the thermoplastic resin sheet and then simultaneously stretching the thus-adhered sheets to form a thin fiber-reinforced sheet in which the direction of stretching is perpendicular to the direction of the fibers.