Hi Toshinao,
If you create an ibd via new diagram dialog you should be forced to create it under Block or Constraint Block already, but if you create the ibd via Model Explorer’s popup menu this checking is omitted.
To move ibd under a Block, right-click on it in Model Explorer, select Sub Diagrams > Existing Diagrams, and then add that ibd to make it sub-diagram of the Block. You can reference section " Add an existing diagram as a sub-diagram" in this documentation for details:
https://www.visual-paradigm.com/tutorials/how-to-use-subdiagrams.jsp
Hope this helps,
Antony.
