To monitor the development of a crack in a wide range of a direction, by measuring the impedances of two extended electrodes constituted so as to pass a current to a resistance layer and the impedance of the resistance layer, and measuring the size of the crack in a processed article or a test piece from the measured values.

Curved electrodes or bus bars 2 are formed on the resistance layer 1 formed on a dielectric substrate 3. A detector is mounted on a processed article or a Lest piece. In the development of the crack of the processed article or the test piece, a crack is generated in the dielectric substrate 3 and the resistance layer 1 in the same pattern as the crack pattern of the processed article or the test piece. When the crack is generated in the resistance layer, the resistance layer 1 is reduced in its cross area to be increased in its resistance. Voltage is dropped between two bus bars 2 by the impedance of the resistance layer 1 and this voltage drop corresponds to the impedance of the resistance layer 1. The voltage drop has the linearity corresponding to a crack size by the curving ratio of the bus bars 2, and is made substantially regardless of a crack developing direction.