1. An image angle variation detection device comprising: an image angle variation detection unit implemented by hardware including a processor, and which detects an image angle variation of an imaging device from videos shot by the imaging device based on fixed point information including the position of a fixed point specified by an image angle of the imaging device and the features indicating the characteristics of the fixed point; a feature point extraction unit implemented by the hardware which extracts a feature point from a video shot by the imaging device; and a fixed point update unit implemented by the hardware, and which updates the fixed point information by evaluating stability based on a number of detections of the feature points within a period, or updates the fixed point information from the detected feature points when there is no change in the position of the feature point between frames during a certain period of time; and a situation determination unit implemented by the hardware and which determines that the image angle of the imaging device is varied when a change in the image angle variation is stable for a certain period of time, wherein the change in the image angle variation being stable for the certain period of time is determined from a change in an image angle variation parameter between frames of videos shot by the imaging device is stable for the certain period of time, and when the image angle variation is in a range in which the imaging device cannot be returned to the non-varied state based on a camera control information, the situation determination unit alerts a user to the varied image angle.