To control the corrosion of a metal in use for a container and others with the use of a proper amount of a neutralizing agent by determining the amount of the agent by measuring the hydrogen ion concentration of water which extracted water soluble acid substances in an acidic nonaqueous solution and/or condensed water.

For example, in coal tar 1 in an acidic nonaqueous solution flowing in transfer piping 9 of copper, a constant amount of it is extracted into a branch line 11 and is added with a predetermined amount of extracting water 3. Next, a mixed liquid is prepared in a line mixer 4, and water soluble acid substances are extracted into water. After that, the mixed liquid is sent to a static separation tank 5 and separated into coal tar 1 and water 3 which extracted the water soluble acid substances, the pH of the water 3 is measured by an automatic pH measuring device 6, and the measured value is input in an addition controlling device 7. The device 7, in order that the pH of the coal tar 1 be a target value, determines the amount of a neutralizing agent 2 and output it to an addition apparatus 8.