To provide a resin composition which comprises a poly-3-hydroxybutyrate-based polymer and/or polylactic acid and an acrylic elastomer and has a large rate of crystallization and excellent impact resistance.

This resin composition is characterized by comprising 1 to 99 wt. % of the poly-3-hydroxybutyrate-based polymer and/or the polylactic acid and 99 to 1 wt. % of the acrylic elastomer and satisfying the following characteristics (a) and (b). (a) The crystal-melting point observed with a differential scanning calorimeter, when heated from room temperature at a temperature-rising rate of 10°C / min, is 130 to 185°C. (b) The weight-average mol. wt. converted into polystyrene and measured by gel permeation chromatography is 20,000 to 3,000,000.