Hello @植松千晴 and welcome to Atlassian Community!
Currently, it's not possible to show the label on the backlog for next-gen projects because the Board settings are not available yet. We have two feature requests related to view labels:
Please, click on vote and watch in order to hear about updates on the development cycle.
UPDATE - October 15, 2019
We appreciate your feedback and your interest in this feature on next-gen projects. I’ve requested an update from our Product Manager within the linked issues above.
Please be aware that to receive updates from this thread, you can just click on “Watch”, the same as on the feature request.
While a +1 within Community is great to show enthusiasm, these +1 don’t make it over to the issues. We do ask if you want to vote for the issue to please use the “Vote” button within the issue to accomplish this. While we would like totally your +1 votes within Community, the best place for them is the Jira issue.