This plate thickness control device is a plate thickness control device for performing plate thickness control of a hot rolling mill provided with a rolling stand. The plate thickness control device is provided with: a thermometer disposed on an entry side of the rolling stand; a difference computing unit for outputting a differential temperature between a lock-on temperature of a rolled plate, measured by the thermometer, and a measured value relating to a part other than a distal end portion of the rolled plate, measured by the thermometer; a tracking unit for transferring the differential temperature from the position of the thermometer to directly below the rolling stand, on the basis of the plate speed of the rolled plate; and a calculating unit for calculating an amount of reduction at the rolling stand on the basis of the differential temperature transmitted from the tracking unit. The calculating unit may include: a proportional differential control unit for implementing proportional differential control with respect to the differential temperature; and a reduction amount computing unit for calculating the amount of reduction on the basis of an output value from the proportional differential control unit.