1. An image processing apparatus, comprising: an input data obtaining section for obtaining input image data; a features extracting section for extracting features of the input image data; at least one of a storage section for storing features of each of reference images and a reference image obtaining section for obtaining the features of the reference image from an external device communicably connected to the image processing apparatus; a similarity calculation section for comparing the features of the input image data with the features of the reference image so as to calculate a similarity between the input image data and the reference image; and a control section for determining whether it is necessary or not to forbid or restrict image processing with respect to the input image data in accordance with whether or not the similarity is not less than a predetermined threshold value, the reference images including a permissible reference image which permits image processing and a restrictive reference image which forbids or restricts image processing, the image processing apparatus further comprising a discrimination information obtaining section for obtaining, from the storage section or the external device, discrimination information for discriminating the permissible reference image and the restrictive reference image from each other, wherein when a maximum value of the similarity calculated by the similarity calculation section is less than the threshold value, the control section permits image processing with respect to the input image data, when a maximum value of the similarity calculated by the similarity calculation section is not less than the threshold value, the control section permits image processing with respect to the input image data in case where a reference image having a maximum similarity is the permissible reference image, and the control section forbids or restricts image processing with respect to the input image data in case where the reference image having the maximum similarity is the restrictive reference image, and the control section forbids or restricts image processing with respect to the input image data in case where a reference image having a maximum similarity calculated by the similarity calculation section is the permissible reference image and the similarity to the restrictive reference image is not less than a second threshold value higher than the threshold value or in case where the similarity to the restrictive reference image is not less than the second threshold value and a difference between the maximum value and the similarity to the restrictive reference image is not more than a predetermined value.