A microscope equipped with an automatic focusing mechanism, comprising: an observation light source; an objective lens for focusing first light emitted from the observation light source on an object to be detected; an observation optical system for imaging the first light which is reflected by the object to be detected and passes through the objective lens; and a focal-point detector for detecting a positional shift of the object to be detected from a focal position of the objective lens, wherein the focal-point detector includes arranged in the following order a focal-point detection light source for emitting focal-point detection light serving as second light, a collimator lens, a light-blocking plate for blocking a portion of the second light, and a half-mirror for transmitting the second light, characterized by further comprising: an area sensor on which is focused the focal-point detection light that is emitted from the focal-point detection light source as a minute spot light, that is focused on the object to be detected via the objective lens, and that returns via the objective lens upon reflection at the object to be detected, and upon reflection by the half-mirror and a region setting unit (19, 19A) which can set an in-focus assessable region of the focal-point detection light acquired by the area sensor to any position on the area sensor, wherein the region setting unit (19, 19A) includes a random access memory configured to store photoreceptor data on the area sensor surface, and wherein the region setting unit (19, 19A) is configured to perform analysis computations on the photoreceptor data in the random access memory so as to assess an in-focus state of the focal-point detection light. an observation light source; an objective lens for focusing first light emitted from the observation light source on an object to be detected; an observation optical system for imaging the first light which is reflected by the object to be detected and passes through the objective lens; and a focal-point detector for detecting a positional shift of the object to be detected from a focal position of the objective lens, wherein the focal-point detector includes arranged in the following order a focal-point detection light source for emitting focal-point detection light serving as second light, a collimator lens, a light-blocking plate for blocking a portion of the second light, and a half-mirror for transmitting the second light, characterized by further comprising: an area sensor on which is focused the focal-point detection light that is emitted from the focal-point detection light source as a minute spot light, that is focused on the object to be detected via the objective lens, and that returns via the objective lens upon reflection at the object to be detected, and upon reflection by the half-mirror and a region setting unit (19, 19A) which can set an in-focus assessable region of the focal-point detection light acquired by the area sensor to any position on the area sensor, wherein the region setting unit (19, 19A) includes a random access memory configured to store photoreceptor data on the area sensor surface, and wherein the region setting unit (19, 19A) is configured to perform analysis computations on the photoreceptor data in the random access memory so as to assess an in-focus state of the focal-point detection light. an area sensor on which is focused the focal-point detection light that is emitted from the focal-point detection light source as a minute spot light, that is focused on the object to be detected via the objective lens, and that returns via the objective lens upon reflection at the object to be detected, and upon reflection by the half-mirror and a region setting unit (19, 19A) which can set an in-focus assessable region of the focal-point detection light acquired by the area sensor to any position on the area sensor, wherein the region setting unit (19, 19A) includes a random access memory configured to store photoreceptor data on the area sensor surface, and wherein the region setting unit (19, 19A) is configured to perform analysis computations on the photoreceptor data in the random access memory so as to assess an in-focus state of the focal-point detection light.