A frame structure for a saddle type vehicle comprising: a pair of pivot frames provided in the left and right, configured to pivotably support a rear arm which supports a rear wheel via a shaft at a rear portion thereof; and a seat rail welded to the rear portion of the pivot frame and extending rearward and upward, wherein each pivot frame is formed by joining opening end edges of left and right plate portions (81A, 81B) mated like a seashell, a seat rail insertion opening is formed at a rear upper portion of the pivot frame by the left and right plate portions (81A, 81B), characterized in that the hole portions extending along an axis of a front portion of the seat rail are provided on a side surface of the pivot frame, the pivot frame and the front portion of the seat rail are welded along a peripheral edge of the hole portion, the hole portions are provided on both the left and right plate portions (81A, 81B), a back stay insertion opening is provided at a position downward of the seat rail insertion opening of the pivot frame, a lower hole portion is provided on a side surface of the back stay insertion opening so as to extend along an axis of the front portion of the back stay, and the pivot frame and the front portion of the back stay are welded along the peripheral edge of the lower hole portion, the frame structure comprising a cross pipe provided between the left and right pivot frames, wherein the cross pipe is provided on an axis of the front portion of the seat rail at a lower front thereof at a position adjacent thereto the frame structure is produced by a process comprising: a first step of welding a front edge of the left and right plate portions (81A, 81B) and an edge between the seat rail insertion opening and the back stay insertion opening; a second step of inserting an assembly of the seat rail and the back stay into the seat rail insertion opening and the back stay insertion opening; a third step of welding the hole portion on the outside in the vehicle width direction of the left and right plate portions (81A, 81B) and the seat rail, and welding the lower hole portion on the outside in the vehicle width direction and the back stay; a fourth step of welding the hole portion on the inner side in the vehicle width direction and the seat rail and welding the lower hole portion on the inner side in the vehicle width direction and the back stay, and a fifth step of welding an upper edge and a rear edge of the left and right plate portions (81A, 81B). a pair of pivot frames provided in the left and right, configured to pivotably support a rear arm which supports a rear wheel via a shaft at a rear portion thereof; and a seat rail welded to the rear portion of the pivot frame and extending rearward and upward, wherein each pivot frame is formed by joining opening end edges of left and right plate portions (81A, 81B) mated like a seashell, a seat rail insertion opening is formed at a rear upper portion of the pivot frame by the left and right plate portions (81A, 81B), characterized in that the hole portions extending along an axis of a front portion of the seat rail are provided on a side surface of the pivot frame, the pivot frame and the front portion of the seat rail are welded along a peripheral edge of the hole portion, the hole portions are provided on both the left and right plate portions (81A, 81B), a back stay insertion opening is provided at a position downward of the seat rail insertion opening of the pivot frame, a lower hole portion is provided on a side surface of the back stay insertion opening so as to extend along an axis of the front portion of the back stay, and the pivot frame and the front portion of the back stay are welded along the peripheral edge of the lower hole portion, the frame structure comprising a cross pipe provided between the left and right pivot frames, wherein the cross pipe is provided on an axis of the front portion of the seat rail at a lower front thereof at a position adjacent thereto the frame structure is produced by a process comprising: a first step of welding a front edge of the left and right plate portions (81A, 81B) and an edge between the seat rail insertion opening and the back stay insertion opening; a second step of inserting an assembly of the seat rail and the back stay into the seat rail insertion opening and the back stay insertion opening; a third step of welding the hole portion on the outside in the vehicle width direction of the left and right plate portions (81A, 81B) and the seat rail, and welding the lower hole portion on the outside in the vehicle width direction and the back stay; a fourth step of welding the hole portion on the inner side in the vehicle width direction and the seat rail and welding the lower hole portion on the inner side in the vehicle width direction and the back stay, and a fifth step of welding an upper edge and a rear edge of the left and right plate portions (81A, 81B).