1. An image capturing system which performs noise reduction processing on an image signal acquired from an image capturing unit, including: a processor, said processor including: a local region extracting unit 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; a first noise reducing unit which performs adaptive noise reduction processing on the target pixel within the local region; a noise estimating unit 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 unit; and a second noise reducing unit 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 unit and the noise amount estimated by the noise estimating unit.