We have performed high-sensitivity magnetic measurements for tensile-deformed SUS304-type austenitic stainless steel to elucidate a relationship between morphology of martensite particles and structure-sensitive magnetic hysteresis properties. A monotonic increase of the volume fraction of martensite phase up to 14.3% through strain-induced martensitic transformation was observed, while coercivity exhibits a maximum at around 4.3% fraction. Measurements of coercivity varying the magnetic-field direction against tensile direction revealed anisotropy in angular dependence of coercivity that becomes weaker with the progress of plastic deformation. These observations were explained as due to a change of the size and anisotropy in shape of martensite phase with deformation.