A conduction breaking device includes a movable member accommodated in an accommodation chamber of a case. A cutter portion projects from an advancing side of the movable member in the moving direction. A gas generator is arranged on the trailing side of the movable member in the moving direction. A conduction member extends to connect a pair of external connection portions and includes a pair of base portions and a breakable portion. The base portions are located in the side wall of the case and extend along the moving direction of the movable member. The breakable portion is shaped to connect the base portions to each other and to block the advancing side of the movable member in the moving direction. Step portions are formed in the breakable portion. Engaging portions that are engageable with the step portions are formed in the case.