expression, pre-noun adjectival
main; principal; chief; leading; important
Formal or literary pre-noun adjectival used to describe the most important or central thing in a group. Often seen in set phrases like 主なる原因 (main cause) or 主なる目的 (main purpose).
主なる原因は資金不足だ。
The main cause is a lack of funds.
主なる目的は情報交換です。
The principal purpose is information exchange.
Derived from the adjective おも (main, important) + なる (archaic attributive form of なり, 'to be'). The form おもなる is a fossilized pre-noun adjectival.