An observation device, comprising an image sensor that forms images of a specimen and outputs an image signal, an AF detection circuit that calculates evaluation values based on the image signal, and a focus control circuit that controls focus position based on the evaluation values, wherein the AF detection circuit calculates a plurality of evaluation values based on signals relating to a plurality of frequency bands of the image signal, and the focus control circuit controls focus position based on maximum value or minimum value of the plurality of evaluation values.