The　collapse　risk　of　building　structures　has　been　one　of　the　major　factors　causing　casualties　and　huge　economic　losses　for　earthquake　disaster　prevention.　This　paper　presents　a　shaking　table　test　on　low-rise　steel　moment　frames　with　consideration　of　mass　irregularity　in　the　elevation　direction.　The　frames　are　subjected　to　naturally　observed　and　artificial　seismic　waves.　As　indicated　from　the　test　results,　the　specimen　with　the　irregularity　of　additive-mass　(additional　5%　of　the　roof　mass)　on　the　top　floor　showed　considerable　amplification　on　the　acceleration　and　drift　responses　at　the　bottom　storey　when　subjected　to　over-design　earthquakes.　A　numerical　model　with　degraded　stress-strain　relation　is　built　in　terms　of　fiber　elements　and　calibrated　by　test　results.　Incremental　dynamic　analyses　are　performed　to　evaluate　the　probabilities　exceeding　three　limit　states　related　to　immediate　occupancy,　life　safety,　and　collapse　prevention.　The　seismic　fragility　curves　through　a　suite　of　near-fault　ground　motions　in　the　Uemachi　area　of　Osaka　are　obtained　for　the　numerical　models　with　and　without　mass　irregularity　on　the　roof,　and　the　vertical　mass　irregularity　tends　to　play　significant　roles　in　the　seismic　design　for　collapse　prevention.