To give up the inheritance rights in Japan, the heir will have to go to the Family Court to make a statement (you will have to make the statement in Japanese language). There is a time limitation of 3 months, so the heir will have to investigate about the property during this period, and make a decision to renounce the inheritance or not.
3 months is definitely not a long period, so if you became somebody’s heir, you will have to move quickly. If you have any questions about Japanese law, you are suggested to consult an attorney (Bengoshi) as early as you can. Also, attorney (Bengoshi) can handle this procedure at the family court in place of you, if you need.
Article 915 An heir shall give unconditional or qualified acceptance, or renunciation, regarding inheritance within three months of the time he/she has knowledge that there has been a commencement of inheritance for him/her; provided that this period may be extended by the family court on the application of an interested party or a public prosecutor.
(2) An heir may investigate inherited property before making an acceptance or renunciation of inheritance.
Article 938 A person who intends to renounce inheritance shall make a statement to that effect to the family court.