A bioinstrumentation apparatus irradiates light onto a measured region of a subject, detects diffused light to acquire internal information on the measured region, and includes: a container holding a light transmitting medium; a light irradiation unit including a plurality of light emitting ends fixed to the container and irradiating a first light and a second light that mutually differ in wavelength onto the measured region that is immersed in the medium; a light detection unit including a plurality of light detecting ends fixed to the container and detecting the diffused light from the measured region; and a computing unit computing the internal information based on an output signal from the light detection unit; the wavelength of the first light being a wavelength at which an absorption coefficient of the measured region and a mean value of absorption coefficient of the medium are substantially equal, the wavelength of the second light being a wavelength at which the absorption coefficient of the measured region is greater than the mean value of the absorption coefficient of the medium, and the computing unit computing the internal information based on an output signal related to diffused light of the first light and computing boundary information between the measured region and the medium based on an output signal related to diffused light of the second light.