I don't speak Japanese myself, so i figured out that you just need to buy all the things you can find in the 4th button submenu and train with this "things" as long as the timeline shows in the menu above this buttons (there is a number between all the japanese that counts down). Don't sell this "things" again when they get to the top menu (took me houres to find this out
) after you have bought and traind everything in this list you can look like this by the 29th-31th of a month.
but until you get there you will have to run the game again and again to get enough money... the last item costs 200.000 of what ever this is (it takes ages...).
i don't know if you can grow even more...
i hope this helped a little