In this paper, a method to measure the stress intensity factor of a two-dimensional crack existing in a member of real structure is presented. The proposed method uses the following properties of piezoelectric materials; (1) isotropic piezoelectric material induces an electric charge which is proportional to the sum of stresses (σ_x+σ_y) on the structural member's surface; (2) the electric charge is also proportional to the integral of (σ_x+σ_y) in the area where the piezoelectric element is adhered to the structure. In order to obtain the stress intensity factors of Mode I and Mode II separately, two small pieces of piezoelectric elements are adhered near the crack tip so that the two piezoelectric elements are placed close to each other with the crack tip positioned between the piezoelectric elements. The electric current from the piezoelectric elements are integrated by the integration circuits and the output voltages which are proportional to the electric charge induced in the piezoelectric elements are measured. The stress intensity factors of Mode I and Mode II of the crack are estimated by the use of the measured output voltages.