If you have been, and still are, successful with futures, fine. It's just that the graveyards are full of those who started out trading futures without years of experience in stocks, where with less risk one can learn to trade and develop a trading plan. Given the large number of stocks that are suitable for trading, there are constant and varied opportunities to quickly gain experience. You must be taking night classes at Sophia as you're day trading.
Most brokers require at least 1 year of experience before they approve a margin account.
If you have 500,000 yen, you're using around 10% for margin, which is fine. But just because you can afford to trade more contracts, isn't a reason to do so. You should stick with 1 contract until you are consistently profitable. Then compound your profits until you can buy 2 contracts. Then 3 etc. But don't forget to withdraw some profits for a rainy day rather than adding contracts.