An infrared light source radiates, to an ATR prism, infrared light in entirety or part of a wavelength range with absorption wavelengths of a biological material. The ATR prism is adherable to a measurement skin. A prism vibration controller is mounted on the ATR prism and vibrates the ATR prism perpendicular to a contact surface between the ATR prism and the measurement skin. A controller causes an infrared photodetector to detect infrared light in synchronization with the vibration of the ATR prism.
