We have investigated the kinetics of successive γ→ε′→α′ martensitic transformation in a SUS304L stainless steel by isothermal holding experiments. An in-situ optical micrograph observation suggests that the γ→ε′ martensitic transformation has so-called isothermal kinetics while the ε′→α′ martensitic transformation has so-called athermal kinetics. The TTT diagram corresponding to the formation of 0.5 vol% of α′-martensite via ε′-martensite from γ-phase shows a C-curve with a nose temperature located at about 103 K, due to the isothermal kinetics of the γ→ε′ transformation.