An imaging apparatus selects and extracts a longest straight line from a captured image, and superimposes the straight line on the captured image to display it by a display device. An orientation detection unit including an acceleration sensor detects a roll angle. An X axis indicating a roll angle 0° and a Y axis indicating a roll angle 90° of the acceleration sensor are superimposed on the captured image, and an original point of the X and Y axes is matched with a center value of the extracted straight line to be displayed by the display device. A user rotates, by a predetermined operation, the image until the X and Y axes match the gradient of the extracted straight line. Thus, a system control unit corrects a roll angle output value of the orientation detection unit by an amount equal to a corrected rotational angle.