1. An image capturing system which performs noise reduction processing on an image signal acquired from an image capturing unit, including: local region extracting means which extracts, from the image signal, a local region including a target pixel to be subjected to the noise reduction processing and at least one neighboring pixel positioned spatially in the vicinity of the target pixel; first noise reducing means which performs adaptive noise reduction processing on the target pixel within the local region; noise estimating means which estimates the noise amount with respect to the target pixel based upon the target pixel value subjected to the noise reduction processing by the first noise reducing means; and second noise reducing means which performs noise reduction processing on the target pixel based upon the target pixel value subjected to the noise reduction processing by the first noise reducing means and the noise amount estimated by the noise estimating means.