Where a prosecutor did not attempt to subpoena a witness until the morning of trial, the record did not establish due diligence in attempting to secure the witness’s presence, and thus the court erred in admitting the witness’s testimony from a previous trial. The witness’s testimony was important, resulting in a read-back request from the juries at each trial, so the reviewing court could not say that the outcome of the trial did not depend upon the jury’s ability to judge the witness’s credibility. Thus, reversal was required.