Microscopic-scale strain distributions in a martensitic steel were visualized during tensile deformation over a wide range, including a necking deformation, using the digital image correlation method on replica films. An inhomogeneous strain distribution was developed from the initial stage of deformation. The strain distribution was successfully visualized even in the late stage of deformation after necking occurred. It was found that the high- and low-strain regions formed at the initial stage of deformation were maintained even during necking deformation. The von Mises equivalent strain in the high-strain region was ten times larger than that in low-strain region. Therefore, it can be concluded that martensitic structure has significant inhomogeneous deformability attributed to its microstructure, and its characteristic hardly changes even in the late stage of necking deformation.