1. A supervising system, for image supervising a supervising area by taking picture images, comprising: an image forming device, configured to take pictures of an image in the supervising area, even in a condition wherein the image forming device is at least one of covered, obstructed, sprayed, and sticked with drops of rain; an illuminance sensor, configured to detect an illuminance in the supervising area; a feature extracting portion, configured to extract a value of a predetermined feature in an image, from image frames of pictures taken by the image forming device; a first time change detector, configured to: define a frame as a scheme frame, when a differential value between frames of the value of the predetermined feature exceeds a prescribed value, and compare a statistical value of the value of the predetermined feature of a group of frames composed of a prescribed number of frames taken after the scheme frame, with a statistical value of the value of the predetermined feature of a group of frames composed of a prescribed number of frames taken before the scheme frame; a second change of time detector, configured to detect change of time of the illuminance as detected; and a scheme judging portion, configured to detect a scheme to affect an imaging of the image forming device, wherein the scheme judging portion is provided with a table concerning a relationship between the change of time in the value of the predetermined feature, or the illumination and an existence or not of the scheme, in order to detect the scheme in accordance with the table; and wherein detection takes into consideration the change of time in the value of the predetermined feature and the change of time in the illuminance as detected; wherein the first time change detector is configured to detect the change of time in the value of the predetermined feature by calculating an integral value of the value of the predetermined feature; and wherein the first change of time detector is configured to detect a change of luminance value of an image as being greater than or equal to a predetermined value, and then to output the detected result to the scheme judging portion, when: a difference between integral values of the predetermined feature during a process ranging from a predetermined number N of frames including the scheme frame to the predetermined number N of frames after the scheme frame, in accordance with the scheme frame, is the prescribed value or more, and the image is divided into a plurality of blocks, and an area revealing the change of luminance value occupies more than half of the plurality of blocks.