A similarity calculation unit calculates a similarity between pixel values of a target pixel and a reference pixel and a similarity between the pixel value of the target pixel and an average value of pixel values of reference pixels, in an input image. A weighting factor calculation unit calculates weighting factors based on the calculated similarities. A weighted average value calculation unit calculates weighted average values of the pixel value of the reference pixel and the average value of the pixel values of the reference pixels by use of the calculated weighting factors. A subtraction unit calculates a difference value between the pixel values of the target pixel and the reference pixel. An isolation degree calculation unit calculates an isolation degree based on the calculated deference value. A correction unit corrects the target pixel based on the calculated isolation degree.