To obtain a filler useful for improving thermal conductivity of a resin product such as a semiconductor sealing resin.

This filler for improving thermal conductivity of a resin product comprises AlN spherical sintered body having 10-200 μm average particle diameter. The filler is obtained by blending AlN powder with a molding adjuvant, wet-blending the obtained mixture, granulating the wet-blended materials by a spray drier to provide granules, mixing BN powder with the obtained granules and firing the obtained mixture in nitrogen stream at ≥1,500°C.