1. A fundus oculi observing device, comprising: an optical system configured to split a light from a light source into a signal light and a reference light and make the signal light propagated through a fundus oculi of an eye and the reference light propagated through a reference object interfere with each other to generate an interference light; a detector configured to detect the generated interference light and generate detection signals; a scanner configured to perform a scan of a target position with the signal light on the fundus oculi; an image forming part configured to form a tomographic image of the fundus oculi based on the detection signals generated during the scan; a specifying part configured to specify a characteristic site of the fundus oculi depicted in the formed tomographic image; and a controller having a determination part configured to determine a positional relation between the characteristic site of the tomographic image specified by the specifying part and a predetermined position within a frame of the tomographic image, the controller being configured to, based on the determination result, control the scanner to change the target position of the signal light so that the characteristic site is depicted within a standard region from the predetermined position within the frame and to perform another scan of the changed target position with the signal light on the fundus oculi, wherein the controller is configured to, when it is determined that the characteristic site is depicted within the standard region, control the optical system and the scanner to scan the target position of the signal light and generate a new interference light, and control the image forming part to form a new tomographic image based on the new interference light; the controller is configured to, when it is determined that the characteristic site is not depicted within the standard region, control the optical system and the scanner to scan the target position with the signal light again and generate a new interference light, control the image forming part to form a new tomographic image based on the new interference light, control the specifying part to specify the characteristic site depicted in the new tomographic image and, based on a position of the specified characteristic site within a frame of the new tomographic image, change the target position of the signal light so that the characteristic site is depicted in a predetermined position within the frame; and the determining part is configured to obtain a displacement of the position of the characteristic site from the predetermined position, determine that the characteristic site is depicted within the standard region when the obtain displacement is less than a predetermined threshold, and determine that the characteristic site is not depicted within the standard region when the displacement is equal to or more than the predetermined threshold.