Apparatus for continuously measuring the width of a continuously cast steel slab moving along a roller table including a pair of measuring rolls slidably mounted to a guide rail, one on each side of the slab, and further including an air cylinder interconnected between the two measuring rolls adapted to bias the measuring rolls towards each other sufficient to bring them into contact with, and roll against, the slab sides while the slab is in motion, so that the distance between the contacting surfaces of the measuring rolls will be a measure of the slab width. The slidably mounted cylinder permits measuring roll contact with a slab that is not travelling on center of a roller table.