PURPOSE: To obtain a mixture suitable as a filler for a heat-conductive formed product by mixing AlN of a sintered aggregate form and AlN of an non-sintered particle form having different average particle size ranges from each other in a specified mixing ratio.

CONSTITUTION: This filler is obtd. by mixing (A) 80 to 50 wt.% aluminum nitride in a sintered aggregate form having 30 to 50 μm average particle size and (B) 20 to 50 wt.% aluminum nitride in a non-sintered particle form having 0.1 to 5 μm average particle size. Then the filler is mixed by 90 to 5 wt.% with a thermoplastic polymer (e.g. epoxy resin) or a structurally crosslinked polymer by 10 to 95 wt.% to produce a resin compounded material for casing. The obtd. resin compounded material for casting has low viscosity although it has a high filler content, and the compounded material can easily be formed. Thus, it is suitable for the production of heat-conductive formed parts and composite material.