PURPOSE: To prevent the breakage of a strip in cold rolling by sending and receiving a sheet wave to and from tire-type ultrasonic-wave probes placed at the specific positions of a running steel strip before cold rolling and detecting flaws in both surfaces of steel strip.

CONSTITUTION: A base sheet 10 for cold rolling is temporarily stored in a strip storing device 23, and is welded to a succeeding base sheet 10' by a welder 22, to perform continuous rolling. Two tire-type ultrasonic-wave probes 1, 1' are provided to the positions above the base sheet 10 and 80W320mm inside from its side ends. The probes 1, 1' send a sheet wave by receiving an electric pulse from ultrasonic flaw detectors 2, 2', and send the sheet wave to the detectors 2, 2' after electrically converting the received sheet-wave signal, to judge the presence or absence of flow after amplifying. When the flaw is detected, is signal is sent to a process computor 25 to calculate immediately the time when the flaw-part arrives at a tandem mill 24 basing on the length of base material 10 in the storing device 23 and the rolling conditions, thereby performing a counter plan such as a reduction of rolling speed.