When the capsule type strain gage is used for strain measurement of cast iron, the shearing force in the spot weld is produced from the difference of the amount of the deformation of the capsule gage and cast iron. And, the actual strain caused in cast iron cannot be accurately measured because this shearing force deforms the spot weld. In this research, the shearing force caused in the spot weld was analyzed taking this deformation into consideration. As a result, it was clarified that a constant ratio existed between the measured strain by gage and the actual strain caused in cast iron. Therefore, the actual strain in cast iron can be estimated from the capsule gage output value and a constant ratio.