PURPOSE: To make it possible to detect an inclusion about 100μm or more efficiently and economically.

CONSTITUTION: Ultrasonic linear-array probes 1 and W pair at the upstream and the downstream, has the point converting function at the test frequencies in 10-100MHz and has a plurality of piezoelectric transducers 11 and 21 in the direction perpendicular to the rolling direction. The adequate pairs of the probes 1 and 2 are arranged in the direction perpendicular to the rolling direction and/or the rolling direction. In the respective pair, the probe 1 on the upstream side performs electronic scanning continuously over the entier scanning range. When a defect is detected, the probe 2 forming the pair on the downstream side is made to scan by the predetermined number of times only in the small range including the detecting position.