PURPOSE: To make high accurate the lock-on, by using the outlet side gaugemeter as the reference gauge detector for the period of lock-on.

CONSTITUTION: At starting the rolling work, RY 48 is being put OFF, and the output of the rolling reduction control output system 44 depending on the outlet side gauge deviation is being produced for the screw down system. At this moment, the output of the rolling reduction control output system 46 is being inputted into the lock-on mechanism 47. The rolling reduction control output system 44 operates the screw down device to open or close until the outlet side deviation makes zero; when the deviation signal hox for the outlet side gauge makes zero, the zero detector 49 causes the lock-on mechanism 47 to memorize the output of the rolling reduction control output system 46 at that moment according to the lock-on command. As a result, the output of the lock-on mechanism 47 becomes equal to the output of the rolling reduction control output system 46, and the output of 47 becomes the input of the system 46, so that the output of the rolling reduction control output system 46 makes zero. Meantime, RY 48 is put ON, and the rolling reduction control output system 44 is disconnected from the screw down system, and the rolling reduction control output system 46 is connected to the screw down system. Hereafter, the gaugemeter system AGC of this invention starts the operation.