Provided is a method of reforming a produced gas to reform tar contained in the produced gas from a gasification furnace. Quick lime comes in contact with a produced gas (3) from a gasification furnace (2) in a reformer (1) such that the quick lime absorbs carbon dioxide in the produced gas to generate calcium carbonate, and reforming reaction of the tar is performed with calcium as a catalyst using heat of reaction emitted during that time. In addition, as the calcium carbonate of the reformer (1) is supplied and burnt in a reforming agent regenerator (8), the quick lime is regenerated and supplied into the reformer (1) again. According to the above configuration, it is possible to provide a method and device for reforming a produced gas that are capable of simplifying equipment and operation thereof.