A safety valve comprises a main body, fixed body, meltable body, movable body, biasing member, movable plug and plug holding means. The fixed body has a small passageway communicating with the interior of a container, and an upper large passageway. The movable plug has a small portion fitting in the upper end of the small passageway of the fixed body, an intermediate portion providing a clearance therearound inside a lower end portion of the large passageway of the fixed body, and a large portion slidably fitting in an intermediate portion of the large passageway of the fixed body. The large rod portion of the movable plug has a passage extending from its upper end to its lower end, and the intermediate portion of the plug is provided with communication passageways having openings in its peripheral surface and communicating with the lower end of the passage.