An online roll profile measuring system for measuring a roll profile during rolling without the effect of an error caused by the change in rolling condition. The system includes a roll grinder unit and a grinding wheel traversing device. The roll grinder unit has a rotary grinding wheel positioned opposite to a work roll for grinding the work roll, a grinding wheel driving device for rotating the rotary grinding wheel, and a grinding wheel feeding device for pressing the rotary grinding wheel to the work roll. The grinding wheel traversing device moves the roll grinder unit in the axial direction of the work roll relative to a guide rail frame fixed on a housing holding the work roll. A roll profile is calculated, simultaneously with grinding of the work roll using the rotary grinding wheel, on the basis of movement amounts of the grinding wheel feeding device and the grinding wheel traversing device. In this system, the amount of displacement of the housing due to the change in rolling condition during measurement of the roll profile is measured by a roll position measuring device, and the roll profile measured during rolling is corrected on the basis of the amount of the displacement of the housing by an information processor.