To provide a carious tooth diagnostic method and a carious tooth diagnostic apparatus for more reliably and easily diagnosing a carious tooth even for a site that could not be measured by the conventional pH measurement system and for distinguishing whether the cariosity is acute or chronic.

A carious section 13a of a carious tooth 13 is coated with a reagent that reacts with hydrogen ions. Reflection light L2 from the carious section 13a, when it is irradiated with light L11, is captured by an optical fiber 3. The value of pH in the carious tooth 13 is obtained based on the respective spectrum states of the illuminating light L11 and the reflected light L2, thus deciding whether the carious tooth 13 is chronic or acute based on the value of pH.