When a displacement between a reference frame of a plurality of images acquired by continuous image pickup and a target frame is less than a first threshold indicating that such a frame is not likely to be affected by occlusion, smoothing is performed on an object area through a morphological operation with a normal process amount. Conversely, when a displacement between the reference frame of the plurality of images acquired by continuous image pickup and a target frame is larger than or equal to the first threshold, smoothing is performed with the process amount of morphological operation being increased than a normal process amount.