A double side welding method used for producing a differential gear for welding a weld part to join a ring gear fitted on a gear case from both sides. The method uses welding distortion caused by welding to improve product accuracy. The method includes welding from both sides of a weld part where a first member and a second member, which are the objects being joined, face each other. The method is performed by first welding, which is performed from one side, and second welding, which is performed from the other opposite side from that first welding. Heat quantity adjustment, that adjusts the quantity of heat input to the weld part for the second welding, is performed after the first welding, and is carried out such that an amount of welding distortion that is the objective arises.