To introduce a steel strip into the processing equipment such as a side trimmer, in a state that deviation in a position in the plate width direction is small even when the tail end part of the steel strip is out of a pay-off reel.

Suction piping sucking the rear surface of the steel strip S is assembled in a sliding table 20 guiding the tip part and the tail end part of the steel strip S, and its opening end and the sucking port of a sucking device K are connected by a suction hose H. A current detector 11a is in connect with the motor 11 of the pay-off reel 1. When the detected value A of the current detector 11a has reached a prescribed value (a value showing a state where the steel strip S is out of the pay-off reel 1) or below, a signal S2 of starting suction is outputted to the sucking device K, and the suction of the steel strip S is started.