A manufacturing method for a composite steel part including manufacturing a first steel part by preparing an intermediate product in which an extra portion is added, and heating the intermediate product to an austenitizing temperature in a carburizing atmosphere to form a carburized layer, cooling the intermediate product at a rate less than a cooling rate at which martensitic transformation is caused and in which the intermediate product is cooled to a temperature equal to or less than a temperature at which structure transformation due to the cooling is completed, heating the intermediate product to an austenitizing range by high-density energy and thereafter cooled at a rate equal to or more than the cooling rate at which martensitic transformation is caused to form a carburized quenched portion, cutting the extra portion of the intermediate product, and welding the first steel part and the second steel part to each other.