A detector detects a first key information piece embedded on one side of one sheet or more than one respective sheets from which image data is read out by a reader, and a second key information piece embedded on the other side thereof or on another sheet. And a comparator 11 compares to each other, the detected first and second key information pieces and judges whether or not these key information pieces are related to each other. Then, a controller permits an output portion to output image data read out from all judgment target sheets if it is judged about all these respective sheets that the first and second key information pieces are related to each other, meanwhile the controller prohibits the output portion from outputting image data read out from one sheet if it is judged about this sheet that those key information pieces are not related to each other.