In case fires break out on the lower deck of a car carrier ship or a ferry, the fuel cell vehicles (FCVs) parked on the upper deck may be exposed to radiant heat from the lower deck. Assuming that the thermal pressure relief device (TPRD) of an FCV hydrogen cylinder is activated by the radiant heat without the presence of flames, hydrogen gas will be released by TPRD to form combustible air-fuel mixtures in the vicinity. To investigate the possibility of this accident scenario, the present study investigated the relationship between radiant heat and TPRD activation time and evaluated the possibility of radiant heat causing hydrogen releases by TPRD activation under the condition of deck temperature reaching the spontaneous ignition level of the tires and other automotive parts. It was found: a) the tires as well as polypropylene and other plastic parts underwent spontaneous ignition before TPRD was activated by radiant heat and b) when finally TPRD was activated, the hydrogen releases were rapidly burned by the flames of the tires and plastic parts on fire. Consequently it was concluded that the explosion of air-fuel mixtures assumed in the accident scenario does not occur in the real world.