PURPOSE: To economically manufacture homogeneous flexible expanded graphite in a short time with small power consumption without raising the temp. of released gas by irradiating microwaves on an interlaminar graphite compound to cause exothermic expansion.

CONSTITUTION: An interlaminar graphite compound obtd. by immersing graphite particles in a highly oxidizing acid soln. or electrolyzing them in an electrolytic soln. contg. acid is put in an electrically insulated suitable microwave irradiator, and microwaves are irradiated on the compound. Joule heat is generated in the compound, said acid, water, etc. introduced into the graphite layers are suddenly gasified by thermal decomposition, and under the pressure of the fluid each gap between the graphite layers is extended and expanded to obtain flexible expanded graphite with high compression formability. Natural flake graphite having about 20W150 mesh particle size is preferably used as said graphite particles, yet kish graphite, thermally decomposed graphite or other graphite may be used.