Technological Research Association of Mega-float is developing pontoon-type very large floating structures (VLFS). This system consists of floater, moorings and breakwater. If the VLFS is used as an airport, many passenger, crews and ground service persons will stay on the VLFS. VLFS also supports many facilities and properties. VLFS must be highly safe and this paper presents a discussion on target safety of VLFS as a structural system. Risk analysis and scenarios of catastrophic failure of pontoon-type VLFS are briefly reviewed. Safety levels of various activities in Japan are investigated in historical context and fatal accidental rate for the activities was calculated. Psychological aspect of acceptable risk when the VLFS is provided for public use is discussed. Acceptable safeties are also estimated based on the equations proposed in the past. A trial calculation was made for an international airport using several methods.