People tend to feel difficulty in working on displays rather than on paper. We suppose that rather long holding time, for our short-term memory, requested by working on a display is a dominant reason of difficulty with complicated tasks on a display. That's because working on a display, on which we often have to watch multi sheet of documents alternatively, generally force us longer memory holding time than when we are working on multi sheet of real papers. Subjects were requested to process tasks which need cross-reference between multi pages of documents with two viewing conditions of the pages; one was parallel showing of the multi pages and another was alternate showing of the multi pages. Measured results showed longer working time in the alternate showing condition. We regard the increase of the working time in the alternate showing condition as a result of shortage of short-term memory ability more eagerly requested by the alternate showing condition.