PURPOSE: To reduce the generation of a breaking defect by a method wherein a breaking device is constituted into such a structure that a clamping force, which is caused by a large-diameter roller and a small-diameter roller, to material insulating substrates is automatically subjected to feedback control on the basis of the result of breaking to two pieces of rod type insulating substrate.

CONSTITUTION: When two pieces of rod type insulating substrate A3 adjacent to each other are not broken along a creased line, this breaking defect of the pieces A3 are detected by a breaking detecting sensor 19. An air-pressure cylinder 20 is actuated on the basis of a detection signal from this detecting sensor 19, the breaking defect of the pieces A3 are excepted from the upper part of an endless belt 14 and at the same time, screw axes 18 are rotated in the direction to increase a clamping force, which is caused by a large-diameter roller 11 and a small-diameter roller 12, by motors 17 and strings 16 are compressed. Accordingly, the clamping force to material insulating substrates is automatically adjusted in such a way that the breaking defect is not generated. Thereby, a labor hour to require to adjust the clamping force can be reduced and at the same time, the breaking defect can be reduced.