An imager is provided having an image-capturing sensor, a focusing detector, and a flicker detector. The image-capturing sensor captures a subject image with a rolling shutter and outputs image data. The focusing detector determines whether a subject image is in focus of the image-capturing sensor using the output image data. The flicker detector detects a flicker in the output image data. The focusing detector uses horizontal data without vertical data from the image data so as to determine whether a subject image is in focus, when the flicker detector detects a flicker.