To provide a highly reliable and highly accurate method for detecting breaks of steel strips by surely detecting breaks of steel strips irrespective of materials of the steel strips.

Two electrodes 13 and 14 are set on the steel strip 11. Pseudo random signals are propagated between the electrodes. Transmission signals and reception signals are correlated. The break of the steel strip is detected from a change of a correlation waveform. Even when original reception signals cannot be distinguished due to pseudo random signals propagating a path other than inside the steel strip 11, the presence/absence of the break in the steel strip 11 is detected by obtaining a difference from the measured result of a steel strip without breaks.