The present invention provides a polymer composition including a first base polymer (A) containing at least a thermoplastic polymer, a second base polymer (B) containing at least a thermoplastic polymer and not having compatibility with the first base polymer (A), and an additive (C) containing at least a substance not having compatibility with any of the first base polymer (A) and the second base polymer (B), the additive (C) being in the form of liquid or slurry at the lower of a pyrolysis temperature of the first base polymer (A) and a pyrolysis temperature of the second base polymer (B). (A), (B) and (C) are phase-separated from each other, and interfaces each located between two of phases of (A), (B) and (C) contacting each other form three-dimensional continuous parallel interfaces.