A vehicle body side part structure comprising: a side sill inner panel forming a vehicle-cabin inner surface of a side sill provided at a lower side part of a vehicle body and extending longitudinally of the vehicle body; a pillar inner panel (41, 42) forming a vehicle-cabin inner surface of a pillar extending upwardly from a front end (13A, 42A, 52A, 66A, 72A) of the side sill; an outer member connected to vehicle-cabin outer sides of the side sill inner panel and the pillar inner panel (41, 42) to thereby define a closed cross section (61, 64) continuing from the side sill up to the pillar; and a bulkhead partitioning the closed cross section (61, 64) into plurality, characterized in that the outer member comprises: a first U-shaped cross section (18A, 23A, 31, 41A, 51, 78) formed into a U shape in cross section by a sill outer panel sidewall opposed to a vehicle width direction and by sill outer panel upper and lower walls extending from upper and lower ends of the sill outer panel sidewall centerward in the vehicle width direction, the first U-shaped cross section (18A, 23A, 31, 41A, 51, 78) defining, jointly with the side sill inner panel, a side sill closed cross section; and a second closed cross section (61, 64) formed into a U shape in cross section by a pillar outer panel sidewall opposed to the vehicle width direction and by pillar outer panel front and rear walls extending from front and rear ends of the pillar outer panel sidewalls centerward in the vehicle width direction, the second U-shaped cross section (18A, 23A, 31, 41A, 51, 78) defining, jointly with the pillar inner panel (41, 42), a pillar closed cross section, the bulkhead comprises: a rear end flange connected to the sill outer panel upper wall (32, 33); a front end flange connected to the pillar outer panel front wall; and a partition formed into a U shape in cross section by the sill outer panel side and lower walls, the partition defining a third U-shaped cross section (18A, 23A, 31, 41A, 51, 78), the third U-shaped cross section (18A, 23A, 31, 41A, 51, 78) defines, jointly with the side sill inner panel, a closed cross section extension (62, 65) extending as far as the pillar outer panel front wall. a side sill inner panel forming a vehicle-cabin inner surface of a side sill provided at a lower side part of a vehicle body and extending longitudinally of the vehicle body; a pillar inner panel (41, 42) forming a vehicle-cabin inner surface of a pillar extending upwardly from a front end (13A, 42A, 52A, 66A, 72A) of the side sill; an outer member connected to vehicle-cabin outer sides of the side sill inner panel and the pillar inner panel (41, 42) to thereby define a closed cross section (61, 64) continuing from the side sill up to the pillar; and a bulkhead partitioning the closed cross section (61, 64) into plurality, characterized in that the outer member comprises: a first U-shaped cross section (18A, 23A, 31, 41A, 51, 78) formed into a U shape in cross section by a sill outer panel sidewall opposed to a vehicle width direction and by sill outer panel upper and lower walls extending from upper and lower ends of the sill outer panel sidewall centerward in the vehicle width direction, the first U-shaped cross section (18A, 23A, 31, 41A, 51, 78) defining, jointly with the side sill inner panel, a side sill closed cross section; and a second closed cross section (61, 64) formed into a U shape in cross section by a pillar outer panel sidewall opposed to the vehicle width direction and by pillar outer panel front and rear walls extending from front and rear ends of the pillar outer panel sidewalls centerward in the vehicle width direction, the second U-shaped cross section (18A, 23A, 31, 41A, 51, 78) defining, jointly with the pillar inner panel (41, 42), a pillar closed cross section, the bulkhead comprises: a rear end flange connected to the sill outer panel upper wall (32, 33); a front end flange connected to the pillar outer panel front wall; and a partition formed into a U shape in cross section by the sill outer panel side and lower walls, the partition defining a third U-shaped cross section (18A, 23A, 31, 41A, 51, 78), the third U-shaped cross section (18A, 23A, 31, 41A, 51, 78) defines, jointly with the side sill inner panel, a closed cross section extension (62, 65) extending as far as the pillar outer panel front wall.