PURPOSE: To easily perform the titled measuring by arithmetic calculations only, by subtracting the load variation due to the roll eccentricity of the top back up roll (TBUR) from the load variation due to the roll eccentricity of the TBUR and bottom back up roll (BBUR).

CONSTITUTION: A TBUR and BBUR are set to a condition where both of the rolls contact with each other and the TBUR is rotated for one revolution, and then, the load of a load cell 1 is sampled at a sampling circuit 4 at every sampling pulse of the rotation and stored in a memory. Then the reference position of the BBUR is adjusted so that the relative phase of the BBUR against the TBUR becomes 90°, 180° and 270° when a mark pulse of the TBUR is generated, and signals from the load cell 1 are stored in memories 7 and 8 for each case after pulses equal to the one revolution of the TBUR are sampled from the mark pulse of the TBUR. Then arithmetic means of memories 5W8 at every same sampling pulse are stored in a memory 12 and the difference between the values of the memories 5 and 12 is stored in a memory 13. The load variation of the BBUR is obtained from the value of the memory 13.