To measure the degree of neutralization of concrete without causing trouble from an aspect of aesthetic appearance by applying a pH indicator developed in color to indicate pH to concrete to measure a degree of neutralization and decoloring the pigment bonded to concrete by a decolorant.

A pH indicator which develops a color capable of being decolored by a decolorant to indicate pH is used in a form of an aq. soln. or ethanol soln. and selected, for example, from thymolphthalein, metacresol purple, cresol red or the like and a plurality of these indicators may be together used. When these indicators are used, pigment is bonded to concrete to damage the surface aesthetic appearance of concrete. Then, the bonded pigment is decolored by a decolorant. As the decolorant, sodium dithionite, sodium hydrogen sulfide or sodium sulfide is used as a reducing agent and sodium hypochlorite, calcium hypochlorite or sodium chlorite is used as an oxidizing agent and these compds. are used as an aq. soln. with appropriate concn.