To provide rear body part structure of a pick up truck capable of improving the rigidity of an opening part fringe to which a lower end part of a back panel, in particular, a load-carrying platform tray is attached to prevent load deformation of the lower end part of the back panel and draining rainwater entering an occupant room side outside the vehicle quickly.

This rear body part structure of the pick up truck is constituted in such a way that an opening part opened and closed freely by an opening and closing panel is formed on the back panel joined with a rear floor panel of the vehicle to separate an occupant room from a load-carrying platform. A flange facing forward or backward and joined with the rear floor panel is extended by a predetermined distance in an end part in the forward and backward directions of a vehicle body of a back panel lower forming an opening lower end fringe of the opening part and constituting a lower end part of the back panel. A front peripheral fringe part of the flange facing forward is upward bent. A drain part passing through onto a lower face side of the rear floor panel is formed at a predetermined position of the flange facing forward. A closed cross section is formed by joining the back panel lower and the rear floor panel mutually.