Hi @kazuki.saito,
Welcome to the Auth0 Community!
I understand that you have encountered issues with logging in to your Application.
After testing this myself, I was able to successfully download the Ruby on Rails Quickstart, run the application, and log in.
When looking closely at your code snippet, I noticed that you are calling the link_to for login. Instead, have you tried using button_to to see if that works?
The button_to is what I am using in the Auth0 Ruby on Rails Quickstart.
I am looking forward to your reply.
Thank you.