Can not access to my own realm's world

What exact message(s) do you see when unable to connect / login?

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it is on Japanese

So I would not know exactly the message in English.

It said,

"joining the realms.

This may take some time."

and after above message, comes back to main screen and said

"unable to access to the Realm. please try to access later"

It has been doing this for all day long.

Right after I typed above message, I was finally able to access the world.

I'm glad it is now working for you. Sometimes there is ongoing server maintenance that prevents connecting. Thank you for letting us know.