To obtain an expanded and extruded biodegradable polymer foamed body by using a biodegradable polymer and methyl formate, which is a non-VOC having environmental compatibility, as foaming agents.

A foaming agent is at least one co-foaming agent and the co-foaming agent is either a physical co-foaming agent (for example, an inorganic agent, carbohydrate, halogenated carbohydrate, carbohydrate having a polar functional group, water or any combination thereof), or a chemical co-foaming agent or a combination thereof. A foaming agent blend may be any combination of methyl formate and one or more co-foaming agents. The polymer foaming agent may contain a biodegradable polymer or a blend of other biodegradable polymers or conventional polymer (non-biodegradable polymer) with the biodegradable polymer. A stable foamed body for various uses including a container and a packing system is obtained by the methyl formate-based foaming agent blend, the same as a heat insulator and a cushioning material for protection. There is provided a manufacturing method of such a foaming agent.