To provide a method and an apparatus for stably improving a fatigue strength in a metallic material by which efficiency of higher fatigue strength than that of the conventional treating method can be obtained.

An ultrasonic impact treatment device provided with an ultrasonic impact treatment means 1 and a cooling means 2, is used to apply the ultrasonic impact treatment to the metallic material 3. In this way, during applying the ultrasonic impact treatment and/or just after applying this treatment, the cooling means 2 is used and the ultrasonic impact treatment portion is cooled at the cooling speed of 20C/sec or higher. Further, if necessary, before applying the ultrasonic impact treatment, the metallic material 3 is preheated to 80-250C.