1. An image processing apparatus, comprising: an input unit to which input images taken in time series are inputted; an image memory in which the input images are accumulated; a fluctuation correcting device; and a parameter control device, wherein the image memory outputs, to the parameter control device, images of a predetermined frame number among the accumulated input images, the parameter control device obtains a comparison-target pixel block size based on the input images and the images outputted from the image memory and outputs the obtained comparison-target pixel block size to the fluctuation correcting device; and the fluctuation correcting device prepares a time-smoothed image by using the input images taken in time series, makes histogram of the comparison-target pixel block size with respect to each pixel of a target image to be corrected in the input images and the time-smoothed image, and corrects the target image by changing a pixel correction amount for each pixel depending on similarity between the histograms.