A synthetic solution containing water, a mixed solvent of a hydrophilic organic solvent and a hydrophobic organic solvent, a polymer stabilizer, a polymerization initiator and an unsaturated monomer is heated, and at least after the heating is started, the pH of the synthetic solution is 5 or less or 9 Provided is a method for producing elliptical, needle-like or rod-like polymer particles satisfying the following (1) to (3) including the step of performing solution polymerization by adjusting as described above. (1) The average (LAV) of the major axis (L) of the projection two-dimensional drawing obtained by irradiating light from the direction orthogonal to the major axis direction is 0.1 to 80 μm, and (2) from the direction orthogonal to the major axis direction. The average (DAV) of the minor axis (D) of the projection two-dimensional drawing obtained by irradiating light is 0.05 to 40 μm, and (3) the aspect calculated from the major axis (L) and the minor axis (D). The ratio (L / D) average (PAV) is 1.5-30.水、親水性有機溶媒及び疎水性有機溶媒の混合溶媒、高分子安定剤、重合開始剤及び不飽和単量体を含む合成溶液を加熱し、少なくとも加熱開始後に合成溶液のｐＨを５以下又は９以上に調整して溶液重合を行う工程を含む下記（１）～（３）を満たす楕円状、針状又は棒状ポリマー粒子の製造方法を提供する。（１）長軸方向と直交する方向から光を照射して得られる投影二次元図の長径（Ｌ）の平均（ＬAV）が０.１～８０μｍ、（２）長軸方向と直交する方向から光を照射して得られる投影二次元図の短径（Ｄ）の平均（ＤAV）が０.０５～４０μｍ、及び（３）前記長径（Ｌ）と短径（Ｄ）とから算出されるアスペクト比（Ｌ／Ｄ）の平均（ＰAV）が１.５～３０。
