Advisor asking me to use my grant to cover his orders
This is delicate issue, and I would advise you to proceed with utmost care. There are a number of ethical, legal, and practical issues to take into account.

First, the legal side - as Bill Barth already mentioned, it is not a given that you even can (easily and legally) move money from your grant to somebody else in your lab. In a nutshell, only the funding agency or the respective support department from your university can help you with this.

Second, in terms of ethical issues, the question arises whether you do in fact "share budget in this lab". In other terms, did the lab head also share his budget with you when you did not yet have a grant? Do think about this critically. In my experience, young independent researchers (myself included) tend to take the support we get from senior researchers for granted sometimes, while being rather protective about our own funding. From your description, it does sound like you yourself are being funded from money acquired by the lab head. In that sense, asking you to give back a small part of your grant to support his research is not necessarily unethical. Further, has he helped you with your grant application? If he has, sharing a bit of your grant with him may be the right thing to do (if even possible, see above).

Thirdly, in practical terms, the question arises whether it is worth for you to fight over this issue. This largely depends on how bad it can end for you if the lab head is really mad at you (in Japan I would assume the answer to this is "very", unfortunately) and how much funding money is concerned. This is another issue that you can really only decide for yourself, but (again), do proceed with caution. You should consider that you not only need to work with this guy day to day, but you may also need his support for your next career step. I have unfortunately seen a few young researchers taking a stand with their mentors "on principle" over relatively minor issues. This kind of thing tends to not go as well as people hope.

All things considered, you would do well to not let your successful grant application go to your head. It is certainly a very important milestone in you career, but don't get into a lone wolf mentality ("I'm so good, I don't need anybody to succeed!").