To secure the necessary groove width of welding while maintaining single pass welding in the butt welding of thick plates.

In the shape of a narrow groove 4 when performing the butt welding of two thick plates 3, the width W0 of an initial groove is set to a value suitable for a material. An angle α1 of a groove surface 4a from the base of the groove to a required thickness area is set to a rather large value in consideration of material-dependent shrinkage by initial welding. By setting the angle α2 of the groove surface 4b of a thickness area in the upper part of the angle α1 to be smaller than α1, a double-step groove is obtained as a whole.