An observation device comprising: a stage on which a container in which an observation target is contained is provided; 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 adapted to move the objective lens in an optical axis direction; a detection section (18, 19) that includes at least one displacement sensor adapted to detect a vertical position of the container provided on the stage; an imaging optical system controller adapted to control the imaging optical system driving section on the basis of the vertical position of the container detected by the detection section (18, 19); a horizontal driving section adapted to move at least one of the stage or the imaging optical system in a main scanning direction in a horizontal plane and a sub-scanning direction orthogonal to the main scanning direction, and to reciprocate the at least one of the stage or the imaging optical system in the main scanning direction; and a scanning controller adapted to control the horizontal driving section, wherein the detection section (18, 19) is adapted to detect the vertical position of the container at a forward position in a movement direction of an observation region with reference to the position of the observation region of the imaging optical system with respect to the container, and to change a position of the displacement sensor in the main scanning direction or the displacement sensor to be used in accordance with a change of the movement direction in the main scanning direction, and wherein the detection section (18, 19) includes a displacement sensor moving mechanism capable of moving the displacement sensor in the main scanning direction between one side and the other side with the objective lens being interposed therebetween, and of moving the position of the displacement sensor from the one side to the other side in accordance with the change of the movement direction in the main scanning direction. a stage on which a container in which an observation target is contained is provided; 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 adapted to move the objective lens in an optical axis direction; a detection section (18, 19) that includes at least one displacement sensor adapted to detect a vertical position of the container provided on the stage; an imaging optical system controller adapted to control the imaging optical system driving section on the basis of the vertical position of the container detected by the detection section (18, 19); a horizontal driving section adapted to move at least one of the stage or the imaging optical system in a main scanning direction in a horizontal plane and a sub-scanning direction orthogonal to the main scanning direction, and to reciprocate the at least one of the stage or the imaging optical system in the main scanning direction; and a scanning controller adapted to control the horizontal driving section, wherein the detection section (18, 19) is adapted to detect the vertical position of the container at a forward position in a movement direction of an observation region with reference to the position of the observation region of the imaging optical system with respect to the container, and to change a position of the displacement sensor in the main scanning direction or the displacement sensor to be used in accordance with a change of the movement direction in the main scanning direction, and wherein the detection section (18, 19) includes a displacement sensor moving mechanism capable of moving the displacement sensor in the main scanning direction between one side and the other side with the objective lens being interposed therebetween, and of moving the position of the displacement sensor from the one side to the other side in accordance with the change of the movement direction in the main scanning direction.