I don't know Japanese law. From what I understand, Bitcoin is completely unregulated and they have a 'hands off' approach. I'm not sure it would be illegal. In the U.S, sure. Please correct me on this.
There seems to be a common misconception about what it means to be "unregulated". It doesn't mean that businesses engaged in the exchange of bitcoins are not subject to any laws.
I am confident that operating an insolvent corporation and engaging in securities fraud (front running, insider trading) are illegal under Japanese law.