PURPOSE: A stabilizer consisting of at least one selected from esters of acrylic or methacrylic acid and monohydric alcohols, glycidyl ethers and so on and of a nitroalkane is used to inhibit the decomposition of Flon in the foaming of a phenolic resin.

CONSTITUTION: In order to inhibit Flon used as a foaming agent for a phenolic resin or its modified product from decomposition, a stabilizer consisting of at least one selected from esters from acrylic or methacrylic acid and a monohydric alcohol, glycidyl ethers, N-methylmorpholine, and aliphatic unsaturated alcohols and of a nitroalkane is used, preferably in an amount of 0.01W5wt%, especially 0.05W1wt% based on Flon to stabilize Flon.