An image defect diagnostic system includes: a memory that stores an image density threshold set for each of partial regions in a test target image being a test target for an image defect and set as a criteria for judging whether or not each of the partial regions has an image defect, and that stores a positional information piece indicating a position of corresponding one of the partial regions in the test target image, while associating the image density threshold and the positional information piece with each other; and a diagnostic unit that compares an image data piece of each of the partial regions generated by scanning the test target image with the image density threshold set for corresponding one of the partial regions stored in the memory, to diagnose whether or not an image defect occurs in the test target image.