A deep portion visualizing endoscope comprises a flexible fiber bundle, into which a laser beam produced by a frequency-sweep single frequency laser beam source is entered. The laser beam is irradiated from a radiating end of the fiber bundle to the region inside of a sample to be viewed, and is reflected from reflection planes of the sample. The reflected laser beam interferes with a laser beam, which has been split from the laser beam before being reflected from the reflection planes and which has traveled by a predetermined optical path length, and an image signal is thereby obtained. The image signal is composed of a plurality of difference-frequency beat signals such that the frequencies, at which the signal intensities repeatedly become high and low, may vary in accordance with the difference between depths of the reflection planes from the inner surface of the sample. A beat signal, the intensity of which repeatedly becomes high and low at a predetermined frequency, is then discriminated from the image signal. An image of a reflection plane at a specific depth is reproduced from the discriminated beat signal.