The thieves used tools to force open the main door to the Haarlem Mill, Derby Road, Wirksworth, sometime between 5pm on Thursday, February 18 and 7.30am this morning, Friday, February 19.
Once they were inside, they searched around before breaking through another door to a wedding manufacturer inside the mill.
It is not yet known what has been stolen following the break in.
Officers investigating the incident are appealing to anyone who may have any information about the incident, or saw a suspicious vehicle in the area to contact them.
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Anyone with any information should call police on 101, quoting crime number 7492/16, or Crimestoppers anonymously on 0800 555 111.