@masakazu mori - For starters, please note that none of the following will not work if you using the Free Plan as all team members in the Free Plan can at least view all boards in the team.

The only way to apply a password to a board, is by setting it’s Share settings to Anyone with the link can view, comment, or edit - this is known as public guest mode. Once you have done this and in order for your other Miro team members to have to use the password, you would have to set the board’s Team access to “No access”:

If you’re goal is only have select team members be able to access the board, then you do not have to password protect the boards. You would still set the board’s Team setting like the above screenshot, but then you could give individual team members team members unique access to the board, e.g., Bill can view, but Sally can edit - you would do this here:

And set their access type:

@Robert Johnson 
Thank you as always for your help!!
It was really easy to understand. 
I will try that.