A conduction breaking device is provided that uses gas from a gas generator to move a cutter block in the thickness direction of a cuttable portion, thereby cutting the cuttable portion between the moving cutter block and a cutting edge portion of a cutting chamber. The cutter block is attached to the cuttable portion such that, before generation of gas by the gas generator, a part of the cutter block is located in the cutting chamber and at a position close to the cutting edge portion in a direction along the surface of the cuttable portion.