PURPOSE: To avoid accident by way of certainly correcting one-sided shift or stopping drive of a conveyor by providing a sensor adjacent to both side edges of a band type expansion part of a conveyor belt and devising it to work by the side edges of the band type expansion part at the time when the conveyor belt running on the bias.

CONSTITUTION: In the case when a band type expansion part 1a is shifted to the right, for example, as an arrow Y, initially an inclined ball piece 13a of a microswitch 13 is pushed down by a side edge 1d of the band type expansion part 1a of a conveyor belt 1, and the inner microswitch 13 closes and transmits a warning signal. And additionally, when the band type expansion part 1a is biased further outward, a side edge 1d of the band type expansion part la pushes an inlined ball piece 14a of the microswitch provided further outside and transmits a danger signal. And a correction roller 15 of a correction mechanism B rolls in both directions is two steps according to different signals transmitted by both detectors A and certainly corrects bias of the band type expansion part 1a.