A side part structure of a vehicle body capable of reliably dispersing a collision load applied from the side of a vehicle is disclosed. A bulkhead arranged in a rocker has an outer bulkhead joined to an outer rocker and an inner bulkhead joined to an inner rocker. Facing walls of the outer bulkhead and the inner bulkhead are provided with proximity portions which are engageable with each other, which allow a collision load at a side collision to be reliably transmitted from the outer rocker to the inner rocker via the proximity portions formed on the mutually facing walls, thereby reliably dispersing the collision load from the side of the vehicle.