PURPOSE: To avoid the operation in the over-load condition or the over voltage condition by discriminating the polarity of a torque command signal in a slave device combined with a mask device for working or shifting a long material through the long material.

CONSTITUTION: In this device, a driving mode detecting circuit 53 for discriminating whether a power running mode or a regenerative mode it is, from the polarity of the torque command value in the slave device is provided. In the case of the power running mode, an over load preventing circuit 54, in which in the case of the power running mode, the velocity command value is as it is until the torque command value exceeds 100%, and the velocity command value is lowered in the prescribed ratio when this value exceeds 100%. In the case of the regenerative mode, an over voltage preventing circuit 55, in which at the time of being the torque command value or lower, the velocity command value is immediately raised in the prescribed ratio, is provided. The change-over circuit 56 between the over load preventing circuit 54 and the over voltage preventing circuit 55, is arranged so as to avoid the over load condition caused by the velocity command value of the slave device being lower than the value decided with running velocity of the long material, or the over voltage condition caused by the slave device being rotated in the running condition of the long material.