PURPOSE: To prevent the floating of a sheet an enable an accurate measurement of the thickness of the sheet by having a pair of guide rolls, which stretch a sheet so that it may stick fast to the periphery on measurement sensor side of a roll for measurement, and a nip roll mechanism.

CONSTITUTION: A pair of guide rolls 51 and 52 stretch a sheet 100 in U shape an cooperation with a roll 10 for measurement so as to stick fast to the periphery on sensor head 20 side of the roll 10 for measurement. Moreover, a nip roll mechanism 40 presses the sheet 100 against the roll 10 for measurement with a nip roll 41, and also approximates or separates the nip roll 41 to or from the roll 10 for measurement. An arm 42 supports the nip roll 41 rotatably on one side, and can rotate around a fulcrum 421 on the other. Moreover, for a hydraulic pressure cylinder 43, the tip of the piston rod is connected to an arm with a pin, so it can rotate the arm by the piston rod reciprocating rectilinearly.