1. An image-capturing apparatus, comprising: a sensor capturing an image of an object which is rectangular; a display displaying a captured image, which is an image captured by the sensor; and a processor configured to: determine, as an output target image, the captured image displayed on the display at a designated timing; detect four groups of edge pixels arranged in a shape of a line segment on the image captured by the sensor; and cause the display to display first missing information indicating that one corner of the object is outside an image area captured by the sensor, in a case where three of vertexes of a quadrangle surrounded by straight lines corresponding to respective four line segments represented by the detected four groups of edge pixels are within the captured image area and a remaining one of the vertexes of the quadrangle is outside the captured image area.