To provide a manufacturing method of a stock for hot/warm plastic working and its plastic working method to solve a problem of lowering of fatigue strength, etc., and strength reliability due to a carbide crack in cold/ warm plastic working.

A manufacturing method of a stock is so that a molten steel containing C/Cr in term of (Cr+15.5C) of 14.0-42.0 mass % is sprayed to droplets, is collected/solidified to be an ingot, a max length of the carbide in the structure is set to ≤5 μm. A plastic working method is so that by using the stock by the manufacturing method, the deformation in elongation or compression having a cross section change rate of ≥15 % is applied to at least to part by cold/warm plastic working at ≤800°C.