To obtain a foamable polymer composition and its foam which has excellent heat resistance, chemical resistance and electric characteristics because the main resin component of the foam is a specific cyclic hydrocarbon-based polymer and which is usable in a wider range than a conventional foam.

This foamable polymer particle is characterized in that a blowing agent is added to a specific cyclic hydrocarbonbased polymer. The foamable polymer particle is characterized in that ≥20 vol.% calculated as an amount in a standard state of a gas having ≥-200°C boiling point or sublimation point is added to a specific cyclic hydrocarbonbased polymer. This foam is characterized in that the foamable polymer particle is expanded 1.05 to 50 times the volume of the foamable polymer particle.