1. A monitoring apparatus comprising: an image capturing unit for capturing an image of an object to be monitored; an inversion condition determining unit for determining whether an inversion condition is satisfied or not, based on the visibility of the image captured by the image capturing unit; a display image generating unit for generating a negative/positive inverted image from the captured image when it is determined by the inversion condition determining unit that the inversion condition is satisfied; an exposure controlling unit for effecting exposure control of the image capturing unit by controlling a gain value or a shutter speed of the image capturing unit; and said inversion condition determining unit determining the inversion condition based on the gain value when the exposure controlling unit controls the gain value, said inversion condition determining unit determining the inversion condition based on the shutter speed when the exposure controlling unit controls the shutter speed.