There is provided an optical tomographic observation device which has a resolving power which is higher than those of a conventional optical tomographic observation device and a confocal microscope by a simple configuration by applying a homodyne phase diversity detection technology and designing so as to satisfy the following formula when λ is a wavelength of a laser light source, Δλ is a wavelength half width at half maximum, NA is a numerical aperture of an objective optical element, S is an effective area of a photodetector, and M is a detection magnification of a detection surface relative to a condensing surface. 1 NA 2 ≤ k 1 0.886 ⁢ λ Δλ λ ( NA ⁢ 1 - NA 2 ) ≤ 0.901 ⁢ S M 2 0.441 ≤ k 1 ≤ 0.750