PURPOSE: To obtain an extremely lightwelight fine powdery azodicarbonamide, useful for producing a rubber foam having uniform and fine bubbles, excellent in heat insulating properties, elasticity, mechanical strength, etc., and capable of retaining various performances at a high level for a long period.

CONSTITUTION: This fine powdery azodicarbonamide has ≤3μm, preferably $ 2.5 p m average particle diameter and ≥3.0m2/g, preferably ≥3.5m2/g BET specific surface area. The azodicarbonamide having the average particle diameter and BET specific surface area within the ranges has a larger volume of emitted gases than that of a conventional one and a decomposition starting temperature as low as about 130-140°C and is capable of advancing the cross- linking and expanding nearly at the same time when used for expanding a rubber composition. The azodicarbonamide is used in an amount within the range of 0.1-40 pts.wt., preferably 0.2-25 pts.wt. based on 100 pts.wt. material to be expanded.