The focus detecting apparatus of the present invention is adapted to detect focusing state of a taking lens by detecting relative positional relation between first and second images formed by luminous bundles from an object which passed through first and second image re-forming lenses, including first series of photosensitive elements receiving the first image, second series of photosensitive elements receiving the second image, first calculating device related to a plurality of portions of the series of elements each having a prescribed number of elements on the first series of photosensitive elements, for calculating a representative value for each portion, a second calculating device related to a plurality of portions of the series of elements each having a prescribed number of elements on the second series of photosensitive elements, for calculating a representative value for each portion correlation calculating device for calculating correlation between representative values of the portions of the series of elements on the first series of photosensitive elements and the representative values of each of the plurality of groups provided by dividing the portions of the series of elements on the second series of photosensitive elements, specifying device for specifying a group having highest correlation out of the correlation; and detecting device for detecting relative positional relation between the portions of the series of elements on the first series of photosensitive elements and the specified groups.