By upgrading to the Premium Plan, you can integrate KokuDoku with Kindle.
First, install the Chrome extension「KokuDoku Kindleインポート」on your PC.
After installing the extension, open it from the top-right corner of the browser tab and log in to KokuDoku.

Check the Chrome extension

The login screen on Chrome
Then open your Kindle Library and import your highlights.

Open your Kindle Library and tap “Notes and Highlights”
After opening the Your Notes & Highlights page on Kindle, open Kindle Import and choose “Save & Sync This Book” or “Save & Sync All Books.” The highlights from Kindle will then be imported into KokuDoku and can be viewed in the app.

Tap “Save and Sync” on add-on screen

You can see them on App
When importing to KokuDoku, if you turn “Auto Sync” ON, your highlights will automatically sync with KokuDoku every 60 minutes.

You can also do it automatically by turning “Auto Sync” on
If Kindle Import is set to auto sync, books that were registered directly in the KokuDoku app can be excluded from syncing from the menu in the app.

To exclude some books, you can set it on menu