To provide a method capable of suppressing cost increase of a vehicle body part and providing the vehicle body part with a light weight and a constitution having a sound silencing effect.

When casting a hollow body 21 of a swing arm 20, bubbled metal pieces 26-29 of light weight members are integrally cast-in. Thus, a light weight swing arm 20 is simply produced without increasing a production process. Further, the bubbled metal pieces 26-29 are integrally incorporated to a hollow part of the hollow body 21 as a reinforcing material, resonance of the swing arm 20 is suppressed as well as toughness of the swing arm is increased.