An imaging device includes an image sensor equipped with imaging pixels disposed in a two-dimensional array and focus detection pixels disposed along a specific direction over part of the array of the imaging pixels. The imaging device categorizes an image pattern indicating a pixel output change along a direction perpendicular to the direction in which the focus detection pixels are disposed based upon the outputs from the imaging pixels present around each focus detection pixel. Then, it determines an image output at the focus detection pixel based upon the output from the focus detection pixel and the outputs from the imaging pixels present around the focus detection pixel through an arithmetic operation method corresponding to the categorized image pattern.