A process for producing a sheet-shaped prepreg includes: a fiber bundle introduction step in which tape-shaped reinforcing fiber bundles are disposed in a multilayer arrangement in the thickness direction and introduced into a die to which a molten thermoplastic resin has been supplied; a fiber bundle thickness reduction step in which each of the tape-shaped reinforcing fiber bundles is widened by means of spreading to reduce the thickness; a resin impregnation step in which the tape-shaped reinforcing fiber bundles are impregnated with the molten resin in the die; and a lamination step in which the plurality of resin-impregnated tape-shaped reinforcing fiber bundles are laminated in the die.