A vehicle body lower part structure includes: a side frame that connects a side sill to a battery pack that is arranged below a floor portion; and a battery frame that supports a battery within the battery pack. A first joint flange that is joined to a case bottom portion of the side frame is arranged such that a second joint flange that is joined to the case bottom portion of the battery frame overlaps an upper side in an upward-downward direction of the first joint flange.