A background image holding unit is configured to hold a background image. A threshold table holding unit is configured to hold a threshold. A previous frame image holding unit is configured to hold a previous frame image. An object extraction unit is configured to perform a background subtraction process of calculating difference values between the background image and a latest image and detecting a pixel whose calculated difference value is equal to or larger than the threshold as an extracted object, and configured to perform a process of judging the magnitude of difference between the background image and latest image. The object extraction unit is configured to update the background image and threshold in accordance with the magnitude of difference between the background image and latest image.