A focus detection device includes a micro lens array having a plurality of micro lenses, a light receiving element array having a plurality of light receiving elements for each micro lens and that receives light rays from a plurality of partial areas in which pupils of an imaging optical system are different from each other, in a plurality of light receiving elements respectively through each micro lens and a focus detection calculation circuit. The device generates at least three signal strings respectively corresponding to images of light rays which have been transmitted through at least three of the partial areas, based on signals output from the plurality of light receiving elements of the light receiving element array. The device obtains, from the at least three signal strings, shift amounts of two signal strings corresponding to two partial areas, and detects a focus adjustment state of an imaging optical system based on the obtained plurality of shifts amounts.