To provide an electric power steering device having a lightweight and highly reliable gear box for housing a reduction gear mechanism.

An electric power steering device includes a gear box 33. The gear box includes a housing member 33A for housing a reduction gear mechanism 30 and a cover member 33B covering an opening of the housing member 33A, and is formed by tightly securing and integrating them with a bolt 33C and a nut. The housing member 33A and the cover member 33B are produced by insertion-molding of resin material, wherein metallic core metals 36A, 36B are inserted into the resin material, and have a bolt hole through which a bolt 33C is inserted. The resin material contains foamed plastic having air bubbles with air bubble diameter of ≥1 μm and ≤50 μm.