PURPOSE: To control the distribution of fiber and to control paper quality by passing light beams through a sheet and obtaining signals that indicate the amplitude of the change of a unit weight, the strength of the weakest part of the sheet, and the size of a flock.

CONSTITUTION: A paper sheet 16 is carried in the direction of an arrow 38, a sensor allows a bundle of light of a light source part 12 to he passed through the paper sheet 16 as diffusion light, and transmission light is focused by a photodiode 30 by a columnshaped body 26 made of sapphire and a lens mechanism 28, thus generating an electrical signal. In this case, the paper sheet 16 is made of flocks with different sizes. Therefore, the signal of a sensor 10 has a frequency determined by the traveling speed of the sheet 16 and the size of the flock and a processing circuit 50 processes and analyzes the signal of the sensor 10 and obtains the average value or DC component of the signal by a channel 52 and obtains an AC component by a channel 54. Then, an operation is made based on a specific equation, thus obtaining the amplitude of the change of a unit weight, the strength at the weakest part of the sheet, and correcting various kinds of conditions of a paper-manufacturing process.