A vehicle body side part structure (10) includes a roof side rail (17) and a quarter pillar (15). The roof side rail extends in a vehicle body forward/rearward direction to form a first hollow structure. In the quarter pillar, a second hollow structure is formed, and an upper end portion is connected so as to cross the roof side rail. In addition, in the quarter pillar, an opening section (71) at which the roof side rail is opened, is closed by a lid part (481) at the upper end portion, in an intersecting part of the roof side rail and the upper end portion of the quarter pillar.