According to an attaching structure of a roof side weather strip, a retainer includes a vehicle upper side wall, a connecting part, and an inboard side wall. A first tip locking part, formed in a tip of the vehicle upper side wall, is engaged in a tip locking groove, formed in a tip of a vehicle upper side wall of the weather strip. The vehicle upper side wall of the weather strip is held by the vehicle upper side wall of the retainer. The inboard side wall of the retainer is allowed to abut on an inboard side wall of the weather strip, and a tip of a second tip locking part formed by bending the tip obliquely in the inboard direction is engaged in the engaging groove, so that the inboard side wall of the weather strip is held by the inboard side wall of the retainer.