The main cause of the accident was carelessness.
主たる目的は、新市場への参入です。
The main objective is to enter new markets.
彼がこのプロジェクトの主たる責任者だ。
He is the principal person in charge of this project.
Derived from the kanji 主 (しゅ, 'main') combined with the classical auxiliary verb たり (たる, attributive form of たり), which functions like modern である. The form 主たる is a fossilized rentaishi used only before nouns.