When a family court has investigated the facts in proceedings for adjudication of domestic relations regarding any of the matters set forth in Appended Table 2, it must give notice to that effect to a party and an interested party intervenor, except when it finds it not to be particularly necessary to do so.
Read this with the articles around it
interested party intervenor
「利害関係参加人」
A person who has joined proceedings for adjudication of domestic relations (家事審判) under Article 42, paragraphs (1) to (3) — as a person who is to be subject to the ruling, or, with the family court's permission or by its order, as a person directly affected by its outcome or qualified to be a party — and who may then perform most of the procedural acts a party can.
Short name set inArticle 42, paragraph (7)