Open the Amazon website in your browser (not the app).
Scroll down to the bottom of the page and check the country/region settings. Make sure it’s set to Japan (Amazon.co.jp).
Also check your language settings in your Amazon account:
Go to Account → Content & Devices → Preferences → Country/Region Settings.
Set your region to Japan.
If the Kindle app keeps opening the wrong store, try:
Sign out and sign back in to the Kindle app with your Amazon.co.jp account.
Make sure your device’s language/region settings are set to Japanese/Japan.