There is a minimum capitilisation requirement of 5M JPY to sponsor a visa. For people starting a company and coming in on a business visa (forget the actual name), you will need at least 5M JPY in your company. This is to ensure that your company has enough money to support you for a year. This amount is very much dependent upon a variety of factors, so it may be more than 5M JPY, as far as I know.

The money is still held by your company and you can own the stock, so it's not like you are "spending" it.

Yes, I don't think this is an official number though. It is almost sufficient (according to specialized lawyers) on its own but not absolutely necessary depending on the person degree, skills and plenty of other things.

edit: After checking a relevant Japan ministry of justice webpage [1], 5m yen is given as an indicator, which is why I suppose it is the value given by most lawyers.

[1]: http://www.moj.go.jp/nyuukokukanri/kouhou/nyukan_nyukan20.ht...