PURPOSE: To obtain the extent of movement without the error due to slip, by detecting the extent of movement of a sheet material on a basis of the quantity of rotation of inverted rollers which are rotated by friction to the sheet material.

CONSTITUTION: A sheet material 5 is get between a driving roller 4, which is rotated by a driving source 1, and plural inverted rollers 62 attached to an external frame 8 through springs 72. Rotations of inverted rollers at left and right ends are transmitted to detecting plates 32 through flexible shafts 10, and quantities of rotation of detecting plates are detected by detctors 22. Thus, even if the sheet material 5 is deflected left or right to cause the slip between the driving roller 4 and the sheet material 5, the inverted roller 62 which is not placed on the sheet material is difficult to generate the slip because this inverted roller 62 is rotated by the friction to the driving roller 4, and the quantity of rotation of this inverted roller is larger. Consequently, the smaller detection quantity is defined as the extent of movement of the sheet material to eliminate the error of the extent of movement due to slip.