1. An image capturing system which performs noise reduction processing on multiple image signals acquired from an image capturing unit in a time-series manner, including: a local region extracting unit which extracts, from the image signals, a local region including a target pixel to be subjected to the noise reduction processing and at least one neighboring pixel positioned both spatially and temporally in the vicinity of the target pixel; a first noise reducing unit which performs noise reduction processing on the target pixel within the local region; a noise estimating unit which estimates a 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; a second noise reducing unit which performs noise reduction processing on the target pixel based upon the noise amount estimated by the noise estimating unit and the pixel values of prior image signals subjected to the noise reduction processing in a time-series manner; and a signal storing unit which stores the target pixel values subjected to the noise reduction processing by the second noise reducing unit as pixel values of the image signals subjected to the noise reduction processing.