To provide an automatic gate controller that reduces a control error as much as possible and permits accurate control without increasing a load on and drive current for gate facilities for a dam or the like.

The gate automatic controller is provided with a controlled variable computing part 101 for computing the target opening degree Gm of a gate and a gate control part 102 for controlling the opening/closing operation of the gate. Furthermore, the automatic gate controller, for operating the gate in the open direction or the closing direction to attain a target opening degree, controls such that it factors an amount Q of excessive operation of the gate to be operated even after the gate has received a halt instruction, issues the halt instruction at the time point when the actual opening degree of the gate becomes Gm-Q during operation in the opening direction, and issues the halt instruction at the time point when the actual opening degree of the gate becomes Gm+Q during operation in the closing direction. With the automatic gate controller as above, an amount Qr of actual excessive operation when the gate is operated during certain gate operation is found, and actual excessive amount Qr of operation is used for an expected excessive amount Q of operation from certain gate operation through the following gate operation, thereby performing the gate control.