A method of welding a first metal plate and a second metal plate that is thicker than the first plate, by a laser beam. The first plate and the second plate are disposed to overlap one another in a thickness direction. In a first laser irradiation step, the laser beam is emitted at the first plate, to form an initial nugget including a front-side nugget portion in the first plate, a back-side nugget portion in the second plate and having a diameter smaller than a diameter of the front-side nugget portion, and an annular flat surface portion existing between the front-side nugget portion and the back-side nugget portion. In a subsequent laser irradiation step, the laser beam is emitted again at the initial nugget after the initial nugget is solidified, thereby increasing the diameter of the back-side nugget portion.