To provide a core loss reduction method based on sizes of crystal grains contained in powder particles.

When the average size of crystal grains in the powder particles of the Fe-Si-based dust core which is constituted by pressing the insulation-coated powder and contains Si component of 0.5 to 8.0 wt.% is d (m) and excitation frequency of the dust core is f (kHz), the crystal grain size d is determined such that the expression (1): 120f-0.82d1,800f-0.67 (0.5f200) is satisfied.