The generation of hydrogen-gas from metallic waste is an important issue for the safety analysis of geological disposal facilities for transuranic (TRU) radioactive waste in Japan. The objective of this study is to clarify the gas-generation behavior of stainless steel and carbon steel in non-oxidizing alkaline synthetic groundwater (pH 12.8 and 10.5) at 30 degrees C simulating geological disposal environments. At pH 12.8, the observed gas-generation rate from stainless steel in the initial period of immersion was 1.0 x 10(2) Nml/m(2)/y and 1.0 x 10 Nml/m(2)/y after 200 days (N represents the standard state of gas at 0 degrees C and 1 atm). At pH 10.5, gas generation was not observed for 60 days in the initial period. At 60 days, the gas-generation observed was 5.0 x 10 Nml/m(2)/y. After 250 days, the gas-generation rate approaches zero. At pH 12.8, the observed gas generation rate of carbon steel in the initial period of immersion was 1.5 x 10(2) Nml/m(2)/y and the gas generation rate began to decrease after 200 days. After 300 days, it was 25 Nml/m(2)/y. At pH 10.5, the gas generation rate in the initial period was 5.0 x 102 Nml/m(2)/y and was 1.0 x 10 Nml/m(2)/y after 200 days.