I want geojson in a smaller area than city.

I solved myself.https://e-stat.go.jp distributed the boundary data used for the census in a shp file and was able to convert it to geojson.Thank you.

_{--Questioner's Comment has been reposted as Community wiki}

Why don't you use overpass-turbo to get node information for a specific area from OpenStreetMap?

overpass-turbo is a web-based OSM data mining tool that requires you to write a dedicated query to retrieve all node data in a particular area like this.

Specifically,

Write the two lines below in the text editor section on the left

node({{bbox}});
out;

Press the Run button on the top (wait a moment)

That's it.

If you choose too wide a range, it will be huge and take a lot of time to process, so it is recommended that you start with a narrow range.