PROBLEM TO BE SOLVED: To provide a technique with which high quality CNTs can be stably produced at a low cost.SOLUTION: A production method of a reused substrate uses a used substrate from a surface of which carbon nanotubes have been peeled off, and includes an initialization process to clean the surface of the used substrate, in which cleaning uses a liquid containing fine bubbles. Also a production method of a catalyst substrate for producing carbon nanotubes, which includes a catalyst layer forming process to make a catalyst for synthesizing carbon nanotubes be carried on the reused substrate obtained by the production method of the reused substrate, and to form the catalyst layer on the reused substrate, is disclosed.
