To efficiently open short fiber agglomerate of rockwool without using a complicated apparatus and to prevent reaggregation of opened rockwool fiber.

Rockwool comprising short fiber agglomerate is fed to a turbulent air flow satisfying the conditions of Reynolds number Re≥200,000 represented by Re=U×D/ν (U: average velocity of fluid; D: pipe diameter; ν: coefficient of kinematic viscosity) and opened. Preferably the inside wall of a pipe for pneumatic transportation of short fiber agglomerate of rockwool by air is equipped with a plurality of projections 6 and/or unevenness 8 and the short fiber agglomerate of rockwool is hit on the projections and/or the unevenness to promote opening. When the short fiber agglomerate of rockwool is mixed with woody fiber and/or inorganic powder and used, the woody fiber and/or the inorganic powder is mixed between the opened rockwool fibers to prevent reaggregation of rockwool fiber.