We have developed a positron microbeam using magnetic lenses based on the commercial scanning electron microscope. The minimum beam diameter was 1.9 μm on target. Two-dimensional image of S parameter was successfully obtained. Using this apparatus, S parameter distribution around the crack tip introduced by a stress corrosion cracking of a stainless steel was obtained. S parameter increases at the further region of the tip of the crack. This shows that vacancy type defects may be generated as crack precursor.