To provide a safety valve which is of a space-saving design, of a simple structure, and cost effective.

The safety valve 2, which is operated by both of positive pressure and negative pressure in internal and external relative pressure of a box portion 24 functioning as a gas passage, comprises: a flat plate portion 11, which constitutes a part of the box portion 24 and in which a main ventilation hole 14 for ventilating air is formed; a main valve plate 12, which is formed in such a size as to cover the main ventilation hole 14 of the flat plate portion 11 and in which sub-ventilation holes 15 for ventilating air are formed inside from a range overlapping the main ventilation hole 14; a sub-valve plate 13, which covers the sub-ventilation holes 15 of the main valve plate 12; a plurality of main guide shafts 19, which are passed through the peripheral portion of the main valve plate 12 and the flat plate portion 11 at equal intervals; a plurality of main springs 20, through which each main guide shaft 19 is passed to urge the main valve plate 12 to the flat plate portion 11 in a contact state; a sub-guide shaft 16, which is passed through the center portion of the main valve plate 12 and that of the sub-valve plate 13; and a sub-spring 17, through which the sub-guide shaft 16 is passed to urge the sub-valve plate 13 to the main valve plate 12 in a contact state.