An observation device comprising: a stage on which a container in which an observation target is contained is placed; an imaging optical system that includes an objective lens for forming an image of the observation target in the container; an imaging optical system driving section configured to move the objective lens in an optical axis direction; a horizontal driving section configured to move the stage in a horizontal plane; a scanning controller configured to control the horizontal driving section to move the stage in a main scanning direction in the horizontal plane and in a direction orthogonal to the main scanning direction, to scan each observation position in the container in an observation region of the imaging optical system; a detection section configured to include at an auto-focus displacement sensor (18a, 18b) that precedently detects a vertical position of the container at the observation position before the observation region reaches the observation position in the container, through scanning in the main scanning direction in the observation region of the imaging optical system; an imaging optical system controller configured to control an operation of the imaging optical system driving section when the observation position is scanned in the observation region of the imaging optical system, on the basis of the vertical position of the container detected by the detection section; and a vertical direction error acquisition section configured to acquire an error between a vertical position of the stage at a time point when the vertical position of the container is precedently detected at the observation position by the auto-focus displacement sensor (18a, 18b) and a vertical position of the stage at a time point when the observation region of the imaging optical system is scanned up to the observation position, wherein the imaging optical system controller is configured to control the operation of the imaging optical system driving section to move the objective lens in the optical axis direction on the basis of the vertical position of the container in the observation position that is precedently detected by the auto-focus displacement sensor (18a, 18b) and the error acquired by the vertical direction error acquisition section, in scanning the observation position in the observation region of the imaging optical system, and wherein the vertical direction error acquisition section includes a table in which each observation position in the container and the error at each observation position are set in advance and are associated with each other. a stage on which a container in which an observation target is contained is placed; an imaging optical system that includes an objective lens for forming an image of the observation target in the container; an imaging optical system driving section configured to move the objective lens in an optical axis direction; a horizontal driving section configured to move the stage in a horizontal plane; a scanning controller configured to control the horizontal driving section to move the stage in a main scanning direction in the horizontal plane and in a direction orthogonal to the main scanning direction, to scan each observation position in the container in an observation region of the imaging optical system; a detection section configured to include at an auto-focus displacement sensor (18a, 18b) that precedently detects a vertical position of the container at the observation position before the observation region reaches the observation position in the container, through scanning in the main scanning direction in the observation region of the imaging optical system; an imaging optical system controller configured to control an operation of the imaging optical system driving section when the observation position is scanned in the observation region of the imaging optical system, on the basis of the vertical position of the container detected by the detection section; and a vertical direction error acquisition section configured to acquire an error between a vertical position of the stage at a time point when the vertical position of the container is precedently detected at the observation position by the auto-focus displacement sensor (18a, 18b) and a vertical position of the stage at a time point when the observation region of the imaging optical system is scanned up to the observation position, wherein the imaging optical system controller is configured to control the operation of the imaging optical system driving section to move the objective lens in the optical axis direction on the basis of the vertical position of the container in the observation position that is precedently detected by the auto-focus displacement sensor (18a, 18b) and the error acquired by the vertical direction error acquisition section, in scanning the observation position in the observation region of the imaging optical system, and wherein the vertical direction error acquisition section includes a table in which each observation position in the container and the error at each observation position are set in advance and are associated with each other.