A focus adjustment apparatus, comprising: a focus detection unit to detect image phase difference information using a light flux having passed through a photographic optical system including a focus lens; a storage unit to store a conversion coefficient for converting the image phase difference information into a defocus amount; and a focus adjustment unit to convert the image phase difference information into the defocus amount using the conversion coefficient and to perform focus adjustment by moving the focus lens according to the defocus amount, wherein the focus adjustment unit converts first image phase difference information detected by the focus detection unit into a first defocus amount using the conversion coefficient and causes the focus detection unit to detect second image phase difference information after having moved the focus lens according to the first defocus amount, converts the second image phase difference information into a second defocus amount using the conversion coefficient, and corrects the conversion coefficient according to the first defocus amount and the second defocus amount and causes the storage unit to store the corrected conversion coefficient, characterized in that the focus detection unit includes a plurality of range-finding areas in a view of the photographic optical system and detects first image phase difference information corresponding to a first range-finding area and third image phase difference information corresponding to a second range-finding area, and the focus adjustment unit converts the first image phase difference information and the third image phase difference information into a first defocus amount using a first conversion coefficient corresponding to the first range-finding area and into a third defocus amount using a second conversion coefficient corresponding to the second range-finding area, respectively, causes the focus detection unit to detect fourth image phase difference information of the second range-finding area after having moved the focus lens by a movement amount according to the first defocus amount, and converts the fourth image phase difference information into a fourth defocus amount using the second conversion coefficient, and corrects the conversion coefficient of the second range-finding area according to the third defocus amount, the fourth defocus amount, and the movement amount, and causes the storage unit to store the corrected conversion coefficient. a focus detection unit to detect image phase difference information using a light flux having passed through a photographic optical system including a focus lens; a storage unit to store a conversion coefficient for converting the image phase difference information into a defocus amount; and a focus adjustment unit to convert the image phase difference information into the defocus amount using the conversion coefficient and to perform focus adjustment by moving the focus lens according to the defocus amount, wherein the focus adjustment unit converts first image phase difference information detected by the focus detection unit into a first defocus amount using the conversion coefficient and causes the focus detection unit to detect second image phase difference information after having moved the focus lens according to the first defocus amount, converts the second image phase difference information into a second defocus amount using the conversion coefficient, and corrects the conversion coefficient according to the first defocus amount and the second defocus amount and causes the storage unit to store the corrected conversion coefficient, characterized in that the focus detection unit includes a plurality of range-finding areas in a view of the photographic optical system and detects first image phase difference information corresponding to a first range-finding area and third image phase difference information corresponding to a second range-finding area, and the focus adjustment unit converts the first image phase difference information and the third image phase difference information into a first defocus amount using a first conversion coefficient corresponding to the first range-finding area and into a third defocus amount using a second conversion coefficient corresponding to the second range-finding area, respectively, causes the focus detection unit to detect fourth image phase difference information of the second range-finding area after having moved the focus lens by a movement amount according to the first defocus amount, and converts the fourth image phase difference information into a fourth defocus amount using the second conversion coefficient, and corrects the conversion coefficient of the second range-finding area according to the third defocus amount, the fourth defocus amount, and the movement amount, and causes the storage unit to store the corrected conversion coefficient.