An image sensing apparatus comprising: an image sensor having a first group of focus detection pixels and a second group of corresponding focus detection pixels and a plurality of image sensing pixels, the first focus detection pixel group including a plurality of first focus detection pixels (SHA, SVC) each including a microlens and a wiring layer and each having one opening (OPHA, OPVC) in the wiring layer shifted with respect to the centerline of the microlens in one direction, and the second focus detection pixel group including a plurality of second focus detection pixels (SHB, SVD) each including a microlens and a wiring layer and each having one opening (OPHB, OPVD) in the wiring layer shifted with respect to the centerline of the microlens in the opposite direction, obtaining a pupil division at the image sensor; and control means configured to execute a focus detection of an object having a luminance distribution in one direction by detecting the relative position of two images formed on the focus detection pixels of the first group and on the focus detection pixels of the second group, the image sensing apparatus being characterized in that: the control means is further configured to execute a thinning readout mode that reads out signal charges from a portion of pixels disposed within a part of pixels of the image sensor, and to control that in said thinning readout mode focus detection pixels from first and second focus detection pixel groups are included in the part of the pixels from which signal charges are read out, wherein said image sensor is divided into blocks (BLOCK_H_(i,j); BLOCK_V_(i,j)) each including a plurality of contiguous pixels arranged in a horizontal direction, and a plurality of contiguous pixels arranged in a vertical direction, and each including one focus detection pixel from the first group and one corresponding focus detection pixel from the second group, and wherein the position of a first focus detection pixel from the first group and of the corresponding focus detection pixel from the second group in each block is shifted between blocks. an image sensor having a first group of focus detection pixels and a second group of corresponding focus detection pixels and a plurality of image sensing pixels, the first focus detection pixel group including a plurality of first focus detection pixels (SHA, SVC) each including a microlens and a wiring layer and each having one opening (OPHA, OPVC) in the wiring layer shifted with respect to the centerline of the microlens in one direction, and the second focus detection pixel group including a plurality of second focus detection pixels (SHB, SVD) each including a microlens and a wiring layer and each having one opening (OPHB, OPVD) in the wiring layer shifted with respect to the centerline of the microlens in the opposite direction, obtaining a pupil division at the image sensor; and control means configured to execute a focus detection of an object having a luminance distribution in one direction by detecting the relative position of two images formed on the focus detection pixels of the first group and on the focus detection pixels of the second group, the image sensing apparatus being characterized in that: the control means is further configured to execute a thinning readout mode that reads out signal charges from a portion of pixels disposed within a part of pixels of the image sensor, and to control that in said thinning readout mode focus detection pixels from first and second focus detection pixel groups are included in the part of the pixels from which signal charges are read out, wherein said image sensor is divided into blocks (BLOCK_H_(i,j); BLOCK_V_(i,j)) each including a plurality of contiguous pixels arranged in a horizontal direction, and a plurality of contiguous pixels arranged in a vertical direction, and each including one focus detection pixel from the first group and one corresponding focus detection pixel from the second group, and wherein the position of a first focus detection pixel from the first group and of the corresponding focus detection pixel from the second group in each block is shifted between blocks. the control means is further configured to execute a thinning readout mode that reads out signal charges from a portion of pixels disposed within a part of pixels of the image sensor, and to control that in said thinning readout mode focus detection pixels from first and second focus detection pixel groups are included in the part of the pixels from which signal charges are read out, wherein said image sensor is divided into blocks (BLOCK_H_(i,j); BLOCK_V_(i,j)) each including a plurality of contiguous pixels arranged in a horizontal direction, and a plurality of contiguous pixels arranged in a vertical direction, and each including one focus detection pixel from the first group and one corresponding focus detection pixel from the second group, and wherein the position of a first focus detection pixel from the first group and of the corresponding focus detection pixel from the second group in each block is shifted between blocks.