A safety device for a pressure vessel comprises an easily-breakable member-having a small thickness and a fixing portion formed in the pressure vessel for fixing the easily-breakable member therein. The easily-breakable member has a top wall portion formed with a breakable portion, and a peripheral wall portion having an end. The fixing portion has a wall portion which extends toward the inside of the pressure vessel such that it defines an opening having a shape complementary to a periphery of the easily-breakable member to permit communication between the inside of the pressure vessel and the outside of the pressure vessel. The wall portion has an inner end. The easily-breakable member is fit into the opening of the fixing portion such that the end of the peripheral wall portion is directed toward the inside of the vessel, and the end of the peripheral wall portion of the easily-breakable member and the inner end of the wall portion of the fixing portion are welded to each other for sealing the opening.