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.