Actually the tax issue comes from payment of dividends on held corporate shares. Payments to shareholders are typically subject to a ~30% tax, with the receiving party then having to pay another ~20% income tax (because this is actual income, and not capital gain - the capital gain happens on MtGox bankruptcy side and may be subject to taxes too).

Thanks for your answers. In my country income from an profit from price difference on crypto trade are not taxable if it involve physical person, not an company or corporation and you might look it up how things are in the Japan, there is an difference what you do as physical person or company. Divindends are of course capital gains what would company had to pay tax, but then in my country you would not pay another tax for that same divindends as an income tax. It would be considered as double tax here. Again, dont know japan tax law.