Capabilities of suppressing erosion caused by liquid droplets and achieving a short operation period and cost reduction are provided. A build-up welding method for a structural material used in an erosive environment includes: removing a portion exposed to the erosive environment from the structural material; and forming a hard layer in a portion of the structural material, from which the portion exposed to the erosive environment is removed, the hard layer being formed by short circuiting transfer gas metal arc welding using a solid wire with a hardness of HV 400 or more.