Abstract: A crank shaft is constructed only at the surface layer of a journal portion with a hard nitride layer, so that the resultant crank shaft is available at a low cost, not so heavy and excellent in durability in comparison with a case employing a hard material for the whole crank shaft. Also, a method of manufacturing a crank shaft according to the present invention employs fluorinating process prior to nitriding process to change a passive coat layer, such as oxide layer on the surface of the journal portion to a fluoride layer, which protects the same surface. Therefore, even when there is space of time between formation of fluoride on the surface of the journal portion and nitriding process, the fluoride layer protects and keeps the surface of the journal portion in a favorable condition, resulting in that re-formation of oxide layer on that surface is prevented.