To detect a correct defocusing amount by eliminating false focusing when there are three or more amounts of shift at which the degree of correlation between a pair of signal data strings output from an image sensor is equal to or more than a predetermined value, concerning a pupil dividing type phase difference detection method.

When false focusing occurs at a first aperture F value, an aperture opening F value that does not cause coincidence of a plurality of defocusing amounts is preliminarily determined. Using the determined aperture opening F value, defocusing amounts def7, def8, def9 are detected and then are compared with defocusing amounts def1, def2, def5 detected by using the first aperture F value. Based on the defocusing amounts def1 and def7 that nearly coincide with each other, a true defocusing amount is determined. By detecting defocusing amounts by using two aperture F values, a true defocusing amount can be reliably determined.

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