What evidence you needWe provide general information about what evidence you need to produce in a lawsuit in court.
Evidence rule in civil cases: burden of proof on disputed facts
Japanese laws on civil cases have evidence rules that disputed facts in allegations of both parties, plaintiff and defendant, need to be proven by evidence when the court renders its judgment according to such facts; and such evidence is usually to be produced by either party who can prevail the case based on any facts supported by such evidence.
For example, Mr. X, alleges that Mr. Y borrowed one million yen from him and Mr. Y denies his borrowing money from Mr. X. Then, Mr. X needs to prove that he lent one million yen to Mr. Y by evidence, such as wire transfer record from Mr. X's bank account to Mr. Y's, Mr. Y's writing of receipt of the money, or any third party's testimony of the delivery of the cash having being witnessed.
Mr. Y may admit that he received one million yen from Mr. X; however, for a rebuttal, Mr. Y may allege that the money was donated to him by Mr. X. Then, Mr. X further needs to prove that Mr. Y received the money on condition that he would repay the same amount (and with some amount of interests) to Mr. X.
If Mr. Y rebuts that he received the money as borrowing from Mr. X but he has repaid to Mr. X, then Mr. Y needs to prove by evidence that he has repaid the one million to Mr. X.