To provide a safety valve capable of easily remarkably changing a spring load of an operation spring from a high-pressure side to a low-pressure side and vice versa with a simple structure.

This safety valve 1 includes: a valve casing 10 including two opening parts 13, 15; a diaphragm 2 for partitioning the inside of the valve casing 10 into a primary chamber 14 and a secondary chamber 16; a diaphragm holder 3 connected to the diaphragm 2; a valve hole 34 for making the primary chamber 14 communicate with the secondary chamber 16; a valve element 5 capable of opening/closing the valve hole 34; and an operation spring 4 received by the diaphragm holder 3 to energize the diaphragm 2, and setting pressure at which the diaphragm 2 reacts with excessive pressure in the primary chamber 14 to open the valve element 5. The valve casing 10 comprises a diaphragm case 11 for housing the diaphragm 2 therein, and a spring case 12 for housing the operation spring 4 therein to support an end of the operation spring 4 serving as a supporting point side, wherein the spring case 12 is screwed and connected to the diaphragm case 11 movably in a spring pressure direction of the operation spring 4.