A base material of a ferrous material is heated to an AC1 point, which is a temperature to cause austenite appearance, or higher, and austenite appears in the base material 1a and 1b (S101). An amount of a strain assuming that an Mf point, which is a temperature where the base material becomes martensite completely, is decreased to be less than room temperature is introduced into the base material (S102). The all ferrous material becoming martensite on the occasion of cooling the ferrous material to room temperature is prevented. The base material is cooled to room temperature at a cooling rate where a line extrapolated a cooling curve of the base material intersects with a region where martensite is produced on the CCT diagram (S103). Austenite remains in the manufactured ferrous material at room temperature.