To provide a speed control method for a roll driving motor suitable for carrying out speed control while suppressing hunting even if a periodic fluctuation is generated in the speed achievement due to aging.

A speed controller for roll driving motor comprises a fluctuation detector 36 for detecting periodical fluctuation in a moving average value from a moving average calculator 40, and a moving average calculator 38 for calculating the average of speed achievements from a speed achievement converter 34, i.e., a moving average value, based on a detection signal from the fluctuation detector 36 wherein the number of samples k required for calculating the moving average value is increased based on the detection signal from the fluctuation detector 36. The incremental control of the number of samples k is performed only when a command for operating a roll driving motor 15 at a constant speed is delivered from a main controller.