A “Hardening Penetration Depth Evaluation” technique using ultrasonic wave is developed, instead of destructive inspection. An induction hardened structure possesses extremely fine(∼10μm) grain size, whereas, an non-hardened structure possesses coarse(∼100μm) grain size. This non-destructive technique measures scattering echo, scattered from boundary of hardened structure and non-hardened structure. The advantage of this technique is that through the visualization, one can measure scattering echo around the material and thus acquire quenching pattern. In addition throughout its resolution, one can measure scattering probability distribution of the ultrasonic wave and obtain hardening penetration depth.