To reduce a vast site area, a huge construction cost, or troublesome monitoring work, and an unneeded electric power required for neutralizing industrial wastewater containing acid or alkali, thereby making a plant compact and reducing a total cost required for the neutralization.

Industrial wastewater is neutralized continuously by using an agitator comprising a rotating body carrying out an oscillating rotational motion, a chemical liquid supply lines each for controlling the discharge quantity and discharge interval of a chemical liquid fed from outside, sensors for grasping a system state and a control device, and a continuous neutralization device equipped with an overflow pipe.