1. An image noise measurement system, which performs a noise estimation on an image, comprising: a storage device, storing a previous image prior to the image; a noise estimator, connected to the storage device, for performing a noise estimation on sub-areas of the image and the previous image and producing a noise estimation index for the sub-area of the image; a distribution calculator, connected to the noise estimator, for calculating a distribution of positive and negative signs of pixel differences in the sub-areas of the image and the previous image that are covered by the noise estimator and outputting a positive sign number and a negative sign number; a confidence generator, connected to the distribution calculator, for producing a confident level index according to the positive sign number and the negative sign number; and a recursive filter, connected to the noise estimator and the confidence generator, for performing a recursive filtering operation on the noise estimation index according to the confident level index so as to produce a noise estimate for the image; wherein the noise estimation index is expressed as: ∑ i , j ⁢ ⁢  P N ⁡ ( i , j ) - P N - 1 ⁡ ( i , j )  , wherein i, j indicated the sub-areas covered by the noise estimator respectively, P N (i,j) indicates a pixel of the sub-area of the image covered by the noise estimator, and P N-1 (i,j) indicates a pixel of the sub-area of the previous image covered by the noise estimator; and wherein the distribution calculator comprises: a first comparator, having a first input terminal to receive the pixel P N (i,j) and a second input terminal to receive the pixel P N-1 (i,j) and producing a first trigger signal when the pixel P N (i,j) is greater than the pixel P N-1 (i,j); and a first counter, connected to the first comparator for counting the positive sign number according to the first trigger signal.