PURPOSE: To prepare the titled foam having excellent flame retardancy by heating a composition obtained by incroporating a heat-decomposable foaming agent, a crosslinking agent, a specified crosslinking assistant, and flame retardants into PE to cause to foam.

CONSTITUTION: A foamable composition is obtained by incroporating a heat-decomposable foaming agent (e.g. an ezodicarbonamide), a crosslinking agent (e.g. dicumyl peroxide), 0.5W2.0pts.wt. polyfunctional monomer (e.g. ethylene glycol dimethacrylate) as a crosslinking assistant, 10W30pts.wt. ethylenebistetrabromophthalimide and 10W60pts.wt. Mg(OH)2 as flame retardants, and not more than 5pts.wt. Sb2O3 as a flame retarding assistant into 100pts.wt. PE. This composition is formed into a plate or a sheet, which is then heated to be crosslinked and foamed.