noun
A person who advocates for peace and opposes war, often based on moral or political beliefs.
彼は熱心な非戦論者で、戦争に反対する活動をしている。
He is a devoted pacifist and is active in anti-war efforts.
平和主義者 is a more common and broader term for 'pacifist' or 'peace advocate', while 非戦論者 specifically emphasizes opposition to war.
Compound of 非戦 (anti-war) + 論者 (advocate, theorist).