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It's answer about my own question.
Finally I managed to get refresh token, however I spent over one day due to no guide about this.
I didn't notice that access token & refresh token comes together because the webpage's title only indicates "download access token" and the access token string itself is way long.(2KB)
It would be better to change it's title from "access token" to "access token & refresh token" or else with simple guide how to use it respectively.