Concentric cylindrical member pair welding method and welding device thereof

A welding device for circumferentially melting and welding a cylindrical member pair in which another cylindrical member is inserted inside a cylindrical member, comprising:

• an energy source that produces energy for melting and welding the pair of cylindrical members;
• an applying unit that applies the energy generated by the energy source to the cylindrical member pair;
• a rotating unit that relatively rotates the applying unit and the cylindrical member pair around an axis of the cylindrical member pair, and rotates an irradiation part of energy applied from the applying unit around the axis;
• and
• an adjusting unit that adjusts the amount of energy applied from the applying unit to the cylindrical member pair in association with a rotation angle of the cylindrical member pair around the axis, wherein
• the applying units are arranged at two or more locations in a circumferential direction on a plane orthogonal to the axis of the cylindrical member pair,
• in the adjusting unit,
• the adjustment of the amount of energy in a specific applying unit which is at least one of the applying units is executed in association with a rotation angle to satisfy a relationship of Pd+Pw>θ, wherein Pd is an output decease rotation angle that decreases the energy amount from a steady energy amount HP applied from the specific applying unit in a welding end process, Pw is an overlap rotation angle at which the irradiation parts around the cylindrical member pair overlap with the steady energy amount HP, and θ is a separation angle between the specific applying unit and another applying unit adjacent to each other in a rotation direction around the axis.