PURPOSE: To obtain a titled device capable of obtaining immediately and accurately the outer diameter of each coil during rolling by a simplified apparatus, by using specific equations for calculating said diameters based on the rotational speeds of unwinding and winding reels, the sheet thicknesses at the inlet and outlet sides of a mill, and coil diameters at the initial stage of unwinding or the diameters at the initial stage of calculation.

CONSTITUTION: Speed signals outputted from the pulse generators PLG(r1), PLG(r2) of reels L1, L2 are transmitted to registers R11, R12 through counters C1, C2 and are stored as rotational speeds ω1, ω2. The sheet thicknesses h1, h2 at the inlet and outlet sides of a mill outputted from digital setters DSW1, DSW2 are inputted into multiplying circuits MUL1, MUL2, and are multiplied to obtain products h1ω1, h2ω2. When the coil diameters D10 are known, these values are inputted into the equations I , II from arithmetic circuits CAL1, CAL2 to obtain coil diameters D1, D2 at the optional time. In case of calculating the initial coil diameter D10 based on the initial conditions and the diameter of a mandrel, the calculation is performed by using the circuits CAL1, CAL2 and the equation III. Thus, accurate and reliable coil diameters are obtained, because the need for pulse generators is eliminated to simplify the device, and forward and backward slips are calculated one by one.