A fusion apparatus is equipped with a fuse, a solid combustion agent, an igniting device, and a spring. The solid combustion agent and the igniting device are accommodated within an inner casing. The inner casing is disposed in the vicinity of, or in contact with, the fuse. The inner casing and the fuse are covered by an outer casing. The spring is disposed between the outer casing and the inner casing and urges the inner casing containing the solid combustion agent therein toward the fuse. When the igniting device receives a current supplied from the outside, the solid combustion agent is combusted by the ignition made by the igniting device and the resulting combustion heat softens the fuse. At this time, since the inner casing is pressing the fuse by means of the spring, the fuse is easily broken.