Hi Kenichiro,
By default, there is currently no way to directly create an Invoice directly from an Estimate. We are working on implementing this functionality to more closely Mirror the QuickBooks Online functionality, so keep a close eye on our release notes for future updates!
https://www.method.me/cs/blogs/releasenotes/
If you were looking to build your own solution for this in the meantime, I would recommend you take a look at how Method Handles the Creation of Invoices from Sales Orders (Look specifically at the New/Edit Sales Order, and Create Invoice screens within the Sales Order App)
Thanks Kenichiro.
Regards,
Ben