japan gives permission as officially papyment as bitcoin.but i do not think japan is the leader in the bitcoin.beacause china has bigger mining projects.so china is the most probably a leader in the field 
No, china bans bitcoin as payment which means they just posses big part of bitcoin. Japan will probably lead as they fully accept bitcoin and will have big projects for it. They will also invest big amount of money for their mining. Right now, japan is the most advanced country when it comes to technology so i am sure that they will take advantage of it when it comes to bitcoin.