PURPOSE: To obtain a blowing agent providing crosslinked polyolefin foam having uniform and fine closed cells by dissolving azodicarbonamide crystal in an organic solvent and abruptly precipitating the crystal in a short time by using a non-solvent such as water.

CONSTITUTION: Widely useful azodicarbonamide(ADCA) crystal is dissolved in an organic solvent such as DMF, maintained at 2-25°C, poured into 1.5-3.5 times as much as water to precipitate the ADCA precipitate. In the operation, precipitation of crystal is completed within one hour to give ADCA for blowing agent of thermal decomposition type capable of producing polyolefin foam having uniform and fine closed cells and high expansion ratio. ADCA obtained by this method has 2.5-3.5 of relative intensity ratio I2/I1 where intensity strength I1 is at 3875cm-1 and intensity strength I2 is at 3620cm-1 in infrared spectrum of diffuse reflection Fourier transformation and 6-25μ average particle diameter.