[Problem] To improve the long-term storage stability of an organic borane-Lewis base complex. SOLUTION: The first composite particle of the present invention contains particles of an organic polymer (B) and an organic borane-Lewis base complex (A) contained in the particles. In addition, the second composite particle of the present invention contains particles of the inorganic compound (B') and an organic borane-Lewis base complex (A) adsorbed on the particles.［課題］有機ボラン－ルイス塩基錯体の長期保存安定性を改善すること。［解決手段］本発明の第１の複合粒子は、有機高分子（Ｂ）の粒子と、前記粒子中に含有された有機ボラン－ルイス塩基錯体（Ａ）とを含む。また、本発明の第２の複合粒子は、無機化合物（Ｂ'）の粒子と、前記粒子に吸着された有機ボラン－ルイス塩基錯体（Ａ）とを含む。
