To provide the subject device simple in structure and intended to smoothly measure the thickness of a sliver being in travel.

This device has a pair of a projected roller and a grooved roller 8 and a sensor element 30 supported so as to be mobile when energized; wherein the grooved roller 8 is radially and immobilely supported and has a bottomed groove extending along the outer circumferential surface, the projected roller 7 lies projected into the groove, forming a grip part together with the grooved roller 8, and a sliver 5 is compressed through the grip part and advanced by the action of the pair of the above rollers; the sensor element 30 also lies projected into the groove and presses the sliver 5 against the bottom of the groove in cooperation with the groove bottom on the upstream side of the grip part when viewed in the advancing direction of the sliver 5 and sets in motion according to the thickness fluctuation of the sliver passing the sensor element and the end of the groove bottom.