1. An image retrieval system for retrieving an image corresponding to a retrieval key among plural images subject to retrieval based on the retrieval key, the image retrieval system comprising: a retrieval subject image storing unit that stores the images subject to retrieval so that respective images correspond to first characteristic information and second characteristic information among semantic characteristic information of an image, physical characteristic information of an image and prominence characteristic information showing prominence of an image; a retrieval key inputting unit that inputs the first characteristic information and the second characteristic information as the retrieval key; a characteristic information converting unit that obtains a remainder of the characteristic information other than any two of the semantic characteristic information, the physical characteristic information and the prominence characteristic information based on the two of the semantic characteristic information, the physical characteristic information and the prominence characteristic information; and an image retrieval unit that retrieves an image subject to retrieval, the image at least being approximate to third characteristic information other than the first characteristic information and the second characteristic information among the semantic characteristic information, the physical characteristic information and the prominence characteristic information, from the retrieval subject image storing unit by the characteristic information converting unit based on the first characteristic information and the second characteristic information, which are inputted by the retrieval key inputting unit.