1. An image processing apparatus that performs noise reduction processing on images inputted on a time-series basis, comprising: a recording section that stores an image to be processed and a past image corresponding to the image to be processed; a pixel extraction section that extracts a plurality of pixels in a first predetermined region including a target pixel in the image to he processed and a plurality of pixels in a second predetermined region of the past image corresponding to the target pixel stored in the recording section; a noise amount estimation section that estimates an amount of noise corresponding to the target pixel, a similitude calculating section that calculates a first similitude between the target pixel in the first predetermined region and pixels peripheral thereto and a second similitude between the target pixel in the first predetermined region and a pixel in the second predetermined region based on the amount of noise; a similitude feature value calculating section that calculates a first feature value according to the first similitude in the first predetermined region and a second feature value according to the second similitude in the second predetermined region; a similitude correcting section that corrects the first similitude and the second similitude based on the first feature value and the second feature value; a filter coefficient calculating section that calculates a filter coefficient corresponding to each pixel of the first predetermined region and each pixel of the second predetermined region based on the first similitude and the second similitude corrected by the similitude correcting section; and a noise reduction section that reduces noise of the target pixel based on the filter coefficient.