Fatigue failure of steel occurs when cracks form and grow in the material’s stress concentration area. In order to understand the relation between stress concentration and crack propagation phenomena, non-destructive evaluation methods that can be related to in-situ measurements around the stress concentration area are necessary. In the present work, we developed a scanning Hall probe microscope (SHPM) equipped in a GaAs film sensor and observed three dimensional magnetic fields at room temperature in air. Medium carbon low alloy steels specimens (JIS, S45C) were used in the experiments. Only the area around the artificial slit had been magnetized and the effect of the magnetization area on the artificial slit was observed.