A method for producing a fiber reinforced plastic comprising a plate body having a plate portion (1 a, 1 b) and a protrusion portion rising from at least one surface of the plate portion (1a, 1b), in which the plate portion (1a, 1b) and protrusion portion comprise plural reinforcing fibers and a matrix resin integrated with the plural reinforcing fibers, which comprises, (a) a laminate preparation step for preparing a prepreg laminate which comprises at least two sheets of prepreg base material each of which comprises a reinforcing fiber sheet comprising plural reinforcing fibers each of which has a length of 10 to 100 mm and which are unidirectionally arranged, and a matrix resin of uncured state impregnated wholly or partially into the reinforcing fiber sheet, are laminated in a condition different in the direction of arrangement of the reinforcing fibers, (b) a molding device preparation step for preparing a molding device for molding the plate body, which comprises one mold having a concave portion for forming the protrusion portion and another mold for forming the plate portion (1 a, 1 b) in company with the one mold, (c) a laminate setting step for setting the prepreg laminate prepared in the laminate preparation step between the molds of the molding device prepared in the molding device preparation step, (d) a plate body forming step for forming the plate body in which the plate portion (1a, 1b) and the protrusion portion are shaped by heating and pressing the prepreg laminate set in the molds in the laminate setting step, wherein each layer of the laminate flows to the direction of laminate surface and when flow to the direction of laminate surface is saturated, to the concave portion, that is, to perpendicular direction or to a direction having a certain angle to the direction of the initial laminate surface, to penetrate into the concave portion, while maintaining a laminate state, then the laminate flows, while maintaining the laminate state, to outer layer direction of the initial laminate surface, the matrix resin is solidified, and (e) a plate body demolding step for taking out the plate body, formed in the plate body forming step, from the molds. (a) a laminate preparation step for preparing a prepreg laminate which comprises at least two sheets of prepreg base material each of which comprises a reinforcing fiber sheet comprising plural reinforcing fibers each of which has a length of 10 to 100 mm and which are unidirectionally arranged, and a matrix resin of uncured state impregnated wholly or partially into the reinforcing fiber sheet, are laminated in a condition different in the direction of arrangement of the reinforcing fibers, (b) a molding device preparation step for preparing a molding device for molding the plate body, which comprises one mold having a concave portion for forming the protrusion portion and another mold for forming the plate portion (1 a, 1 b) in company with the one mold, (c) a laminate setting step for setting the prepreg laminate prepared in the laminate preparation step between the molds of the molding device prepared in the molding device preparation step, (d) a plate body forming step for forming the plate body in which the plate portion (1a, 1b) and the protrusion portion are shaped by heating and pressing the prepreg laminate set in the molds in the laminate setting step, wherein each layer of the laminate flows to the direction of laminate surface and when flow to the direction of laminate surface is saturated, to the concave portion, that is, to perpendicular direction or to a direction having a certain angle to the direction of the initial laminate surface, to penetrate into the concave portion, while maintaining a laminate state, then the laminate flows, while maintaining the laminate state, to outer layer direction of the initial laminate surface, the matrix resin is solidified, and (e) a plate body demolding step for taking out the plate body, formed in the plate body forming step, from the molds.