The invention relates to an internal body observation device which includes an ordinary observation light source, an ordinary observation irradiation optical system, a special observation light source, a special observation irradiation optical system, an observation optical system for transmitting the light from an observation target, a detection means configured to detect the light transmitted from the observation optical system as a detection signal, and a processing unit for separating the detection signal in the respective light sources and making the detection signal into images, wherein the light from any one of the ordinary observation light source and the special observation light source is modulated, and the modulated light can be radiated to the observation target together with the light from the other light source, and the detection signal detected by the detection means is separated by the processing unit based on the frequency of the modulation.