An observation device comprising: an imaging optical system having an imaging lens configured to form an image of an observation target in a plurality of containers in which the observation target is contained, wherein each container is a well of a well plate; an operation section configured to perform at least one of a first operation of changing a focal length of the imaging optical system, a second operation of moving the imaging lens in an optical axis direction, or a fourth operation of moving the container in the optical axis direction; a horizontal driving section configured to move at least one of the container or the imaging optical system in a horizontal plane; a scanning controller configured to control the horizontal driving section and moves at least one of the container or the imaging optical system to scan an observation position (R0-R3) in the container; and an auto-focus controller configured to control the operation section and performs an auto-focus control for each observation position (R0-R3), wherein the auto-focus controller is configured to, for each observation position (R0-R3), detect, during the scanning movement, whether the observation position (R0-R3) has reached the boundary portion between the adjacent containers in a scanning direction of the observation position (R0-R3), and switch a start timing of the auto-focus control depending upon whether the observation position (R0-R3) has reached the boundary portion between the adjacent containers. an imaging optical system having an imaging lens configured to form an image of an observation target in a plurality of containers in which the observation target is contained, wherein each container is a well of a well plate; an operation section configured to perform at least one of a first operation of changing a focal length of the imaging optical system, a second operation of moving the imaging lens in an optical axis direction, or a fourth operation of moving the container in the optical axis direction; a horizontal driving section configured to move at least one of the container or the imaging optical system in a horizontal plane; a scanning controller configured to control the horizontal driving section and moves at least one of the container or the imaging optical system to scan an observation position (R0-R3) in the container; and an auto-focus controller configured to control the operation section and performs an auto-focus control for each observation position (R0-R3), wherein the auto-focus controller is configured to, for each observation position (R0-R3), detect, during the scanning movement, whether the observation position (R0-R3) has reached the boundary portion between the adjacent containers in a scanning direction of the observation position (R0-R3), and switch a start timing of the auto-focus control depending upon whether the observation position (R0-R3) has reached the boundary portion between the adjacent containers. detect, during the scanning movement, whether the observation position (R0-R3) has reached the boundary portion between the adjacent containers in a scanning direction of the observation position (R0-R3), and switch a start timing of the auto-focus control depending upon whether the observation position (R0-R3) has reached the boundary portion between the adjacent containers.