Austenitic stainless steels have recently been attracted as a structural material in a hydrogen environment. When transforming from austenite to martensite phase during plastic deformation, i.e., strain-induced martensitic transformation, the transformation triggers microscopic damage such as microcracks, i.e., hydrogen embrittlement, resulting in macroscopic degradation of the mechanical property, whose mechanism has not been unclear. One of the reasons, especially regarding the effects on the microcrack initiation and subsequent propagation, is quite challenging to correlate comprehensively the cracking and the strain-induced martensitic transformation, which has a three-dimensional complex shape and crystal orientation. Therefore, it needs to track the strain-induced martensitic transformation process by three-dimensional in situ observation to reveal the elementary process because the cracking process related to the transformation changes every moment during the deformation. The present study aims to visualize the strain-induced martensite phase by 3D in situ observation by introducing a combination of X-ray CT and X-ray diffraction (XRD).