To enable concrete neutralization depth to be measured accurately through a simple job of boring a small-diameter hole in a concrete skeleton, without decreasing the strength of the concrete skeleton.

A hole is bored in a concrete skeleton, and an aqueous solution of phenolphthalein is sprayed into the hole. As the wall surface of the hole is illuminated, it is scanned and the boundary of a color formed on the wall surface by spraying of the aqueous solution of phenolphthalein is read, this read data being processed and stored. An image is displayed using the stored data, and concrete neutralization depth is measured on the basis of the boundary of the color on the image displayed.