Device for measuring fibre properties in a flowing suspension. The device includes a measuring cell (10) in which there is a measuring field (48) defined between two limiting surfaces and a means (56) of adjusting the width of the measuring field. The limiting surfaces have two opposing, transparent sections (60, 62) that allow illumination through the flowing suspension passing through and measurement by optical means. In addition, the measuring cell (10) has an inlet opening (38) intended for the whole of the suspension flow and an outlet opening (40) intended for the whole of the suspension flow.