A manufacturing method for a composite steel part formed by welding a plurality of steel parts to each other, comprising: manufacturing a first steel part, which includes a carburized quenched portion which has been subjected to a carburizing quenching hardening process and a welding expected portion to be welded to at least a second steel part, by preparing an intermediate product in which an extra portion, which has a thickness equal to or more than that of a carburized layer to be formed in a subsequent carburizing step, has been added to the welding expected portion, and performing the carburizing step in which the intermediate product is heated to an austenitizing temperature or more in a carburizing atmosphere to form the carburized layer on a surface of the intermediate product, a quenching step in which a desired portion of the intermediate product is caused to form the carburized quenched portion, and a cutting step in which the extra portion of the intermediate product is cut; and then performing a welding step in which at least the second steel part is brought into abutment with the welding expected portion of the obtained first steel part to weld the first steel part and the second steel part to each other, characterized in that the carburizing quenching hardening process includes a cooling step, subsequent to the carburizing step and prior to the quenching step, in which the intermediate product is cooled at a cooling 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, and the quenching step includes heating the desired portion of the intermediate product, which is a portion other than the welding expected portion, to an austenitizing range by high-density energy and thereafter cooling the desired portion at a cooling rate equal to or more than the cooling rate at which martensitic transformation is caused to form the carburized quenched portion in the desired portion. manufacturing a first steel part, which includes a carburized quenched portion which has been subjected to a carburizing quenching hardening process and a welding expected portion to be welded to at least a second steel part, by preparing an intermediate product in which an extra portion, which has a thickness equal to or more than that of a carburized layer to be formed in a subsequent carburizing step, has been added to the welding expected portion, and performing the carburizing step in which the intermediate product is heated to an austenitizing temperature or more in a carburizing atmosphere to form the carburized layer on a surface of the intermediate product, a quenching step in which a desired portion of the intermediate product is caused to form the carburized quenched portion, and a cutting step in which the extra portion of the intermediate product is cut; and preparing an intermediate product in which an extra portion, which has a thickness equal to or more than that of a carburized layer to be formed in a subsequent carburizing step, has been added to the welding expected portion, and performing the carburizing step in which the intermediate product is heated to an austenitizing temperature or more in a carburizing atmosphere to form the carburized layer on a surface of the intermediate product, a quenching step in which a desired portion of the intermediate product is caused to form the carburized quenched portion, and a cutting step in which the extra portion of the intermediate product is cut; and the carburizing step in which the intermediate product is heated to an austenitizing temperature or more in a carburizing atmosphere to form the carburized layer on a surface of the intermediate product, a quenching step in which a desired portion of the intermediate product is caused to form the carburized quenched portion, and a cutting step in which the extra portion of the intermediate product is cut; and then performing a welding step in which at least the second steel part is brought into abutment with the welding expected portion of the obtained first steel part to weld the first steel part and the second steel part to each other, characterized in that the carburizing quenching hardening process includes a cooling step, subsequent to the carburizing step and prior to the quenching step, in which the intermediate product is cooled at a cooling 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, and the quenching step includes heating the desired portion of the intermediate product, which is a portion other than the welding expected portion, to an austenitizing range by high-density energy and thereafter cooling the desired portion at a cooling rate equal to or more than the cooling rate at which martensitic transformation is caused to form the carburized quenched portion in the desired portion.