To control the pH value of vegetable printing ink with high efficiency and precision by automating the control of the pH value so as to enable using of the ink friendly to environments.

In the circulation/supply system for the printing ink to be circulated through a diaphragm pump 1, an ink fountain 2 and a subtank 3, voltage which can be converted in terms of the pH value of the ink is measured using a pH sensor 10 and the measurements are compared to a set value including a tolerance range in which the measurements are previously set. When the measurements are outside the tolerance range of the set value, a solution for increasing or decreasing the pH value of the printing ink is automatically added to the circulation/supply system for the printing ink from a tank 12.