Searching for meta key in the forum I found Alt key not working in emacs - #9 by Sabex.
So go to Terminal settings (Alt-,), make sure “Use left Option as Meta key” (左オプションキーをメタキーとして使う, if I copied it correctly from the catalog) is checked and you can use your left Windows key (or whatever is assigned to left Option in your keymap) as Meta.