To examine actual time biological substance such as bacterial plaque and examine bacterial plaque, which cannot be judged with only colony form, by providing an electromagnetic wave absorption intensity detecting means and a prescribed data processing means.

An electromagnetic wave absorption intensity detecting means in a biological examination device has an electromagnetic wave emitting means 1, and imparts prescribed wavelength and wavelength range (half-value width) to the electromagnetic wave emitted from its globar light source by a wavelength selecting element 2. The resulting electromagnetic wave is passed to an expansion angle regulating means 5, and the electromagnetic wave transmitted by a sample and subjected to absorption thereby is incident on an electromagnetic wave detecting means 7. First and second absorption intensities of voltage wave are outputted from the electromagnetic wave detecting means 7, and displayed on a screen or printed on a paper. The result is analyzed by, for example, dental caries risk judging means as diagnostic means to perform a pathogenic diagnosis for dental caries risk. When the measuring wavelength is switched, a second wavelength selecting element 3 is switched by a wavelength selecting means 4.