A method for treating a Cu thin sheet, comprising the steps of: supplying a slurry in which a diffusion bonding aid and a reinforcing material are dispersed in a solvent to a predetermined portion on a Cu or Cu base alloy thin sheet, drying the supplied slurry, and applying a laser to induce melting, solidification, and fixation, so as to form a buildup layer, wherein: (a) a Ni or Ni-Cr alloy powder is used as the diffusion bonding aid; and (b) with respect to the reinforcing material: (i) a carbide base metal compound, a nitride base metal compound, or a boride base metal compound is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 80:20 to 50:50; or (ii) a stainless steel alloy, a hastelloy Ni base alloy, or a stellite Co base alloy is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 50:50 to 1:99; or (iii) Zr-Cu-Al-Ni Zr base alloy is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 50:50 to 5:95, and (c) the median diameters D 50 of both the diffusion bonding aid and the reinforcing material fall within 0.1 to 100 μm, the median diameter D 50 of the diffusion bonding aid is larger than the median diameter D 50 of the reinforcing material, and both the distribution ratio D 90 /D 10 of the diffusion bonding aid and the distribution ratio D 90 /D 10 of the reinforcing material are 4.0 or less. supplying a slurry in which a diffusion bonding aid and a reinforcing material are dispersed in a solvent to a predetermined portion on a Cu or Cu base alloy thin sheet, drying the supplied slurry, and applying a laser to induce melting, solidification, and fixation, so as to form a buildup layer, wherein: (a) a Ni or Ni-Cr alloy powder is used as the diffusion bonding aid; and (b) with respect to the reinforcing material: (i) a carbide base metal compound, a nitride base metal compound, or a boride base metal compound is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 80:20 to 50:50; or (ii) a stainless steel alloy, a hastelloy Ni base alloy, or a stellite Co base alloy is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 50:50 to 1:99; or (iii) Zr-Cu-Al-Ni Zr base alloy is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 50:50 to 5:95, and (a) a Ni or Ni-Cr alloy powder is used as the diffusion bonding aid; and (b) with respect to the reinforcing material: (i) a carbide base metal compound, a nitride base metal compound, or a boride base metal compound is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 80:20 to 50:50; or (ii) a stainless steel alloy, a hastelloy Ni base alloy, or a stellite Co base alloy is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 50:50 to 1:99; or (iii) Zr-Cu-Al-Ni Zr base alloy is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 50:50 to 5:95, (i) a carbide base metal compound, a nitride base metal compound, or a boride base metal compound is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 80:20 to 50:50; or (ii) a stainless steel alloy, a hastelloy Ni base alloy, or a stellite Co base alloy is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 50:50 to 1:99; or (iii) Zr-Cu-Al-Ni Zr base alloy is used as the reinforcing material, and the weight ratio of the diffusion bonding aid to the reinforcing material is specified to be 50:50 to 5:95, (c) the median diameters D 50 of both the diffusion bonding aid and the reinforcing material fall within 0.1 to 100 μm, the median diameter D 50 of the diffusion bonding aid is larger than the median diameter D 50 of the reinforcing material, and both the distribution ratio D 90 /D 10 of the diffusion bonding aid and the distribution ratio D 90 /D 10 of the reinforcing material are 4.0 or less.