PURPOSE: To detect the meandering quantity, catenary quantity and strip shape at the same time regardless of the width size, etc., by applying laser beam to the travelled metal strip from a detecting instrument reciprocatively shifted in the width direction of a furnace and calculating from this reflected beam and the position.

CONSTITUTION: In the horizontal furnace travelling the steel strip S in the furnace 1 through carrying rolls 2, slit 3 is arranged in the width direction at the upper part of the furnace 1. Above this slit, the detecting instrument 4 providing a laser beam oscillator 5 and a beam receiver 6 is arranged and reciprocatively shifted along the slit 3 in the center position between the carrying rolls in the furnace through a screw shaft 7 rotated by a driving motor 8. The laser beam irradiates the steel strip S in the furnace from this detecting instrument 4 through the slit 3 to detect the reflected beam. From this detected signal and a positional signal in the width direction of the furnace with a positional detector 9 fitted to the motor 8, the meandering quantity, catenary quantity and strip shape S are detected by calculation in an arithmetic device 10.