Provided is a method of controlling operation of a tandem rolling mill which enables large reduction rolling in the latter-stage stand of the tandem rolling mill necessary for manufacturing fine-grained steel, and so on. The method comprises: a step of determining a first exit-side sheet thickness in rolling a constant portion of a material to be rolled; and a step of determining a second exit-side sheet thickness in rolling a front end portion of the material, such that a pre-tightening load becomes a set value or less; the material is rolled into the second exit-side sheet thickness, until the front end portion is fed into the stands; the constant portion is rolled by the stand given a pre-tightening load into the first exit-side sheet thickness; and the second exit-side sheet thicknesses of the stands given the pre-tightening load are made larger than the first exit-side sheet thickness.