To omit an inspection of a break state after a metallic component with a through-hole is broken in the through-hole with a mandrel and a wedge.

A breaking method for the metallic component W breaks the metallic component W into two parts by allowing a half-divided type mandrel 12a, 12b to fit in the through-hole H 2 of the metallic component W and driving a wedge 14 between divided parts 12a, 12b of the mandrel. The breaking method includes steps of: detecting a strain caused in the wedge 14; and estimating the break state of the metallic component W based on the corresponding relation among the detected change of the strain caused in the wedge 14, the preliminarily obtained change of strain in the wedge 14, and the break state of the metallic component W. Since the break state of the metallic component W is estimated by the change of strain of the wedge 14 which is being broken, the inspection of the break state of the metallic component after breakage is not required.