1. An image processing apparatus for correcting a defective portion in image data having a plurality of pixels, the apparatus comprising: a first target pixel selecting section for selecting a plurality of first target pixels one after another from normal pixels; a second target pixel detecting section for detecting, for each first target pixel selected by said first target pixel selecting section, a defective pixel present within a predetermined area extending around said first target pixel as its center and then detecting a normal pixel present on an extension line connecting said first target pixel to said defective pixel and setting this detected normal pixel as a second target pixel; an interpolation value calculating section for calculating for one or more defective pixels present between said first target pixel and said second target pixel, an interpolation value for each said defective pixel based on a pixel value of said first target pixel and a pixel value of said second target pixel; a weight coefficient calculating section for calculating a weight coefficient for each pair of said first target pixel and said second target pixel, based on the pixel value of the first target pixel and the pixel value of the second target pixel; a memory for storing a plurality of interpolation values calculated for each defective pixel by said interpolation value calculating section and the weight coefficient calculated for each pair of the first target pixel and the second target pixel by said weight coefficient calculating section as said first target pixel selecting section selects the plurality of first target pixels; a weighted average value calculating section for calculating a weighted average value of the interpolation values of each defective pixel by using the plurality of interpolation values stored in said memory for each defective pixel and the weight coefficient also stored in said memory for each pair of the first target pixel and the second target pixel; and a correcting section for correcting each defective pixel by using said weighted average value calculated by said weighted average value calculating section.