NOVELTY - Manufacture of copper-gallium alloy slab involves preparing copper and gallium in crucible (1), heating crucible, obtaining molten metal of copper-gallium alloy in crucible, pouring obtained molten metal in crucible into casting mold (2), cooling and coagulating the poured molten metal in casting mold at room temperature, and molding into shape of slab. The casting temperature and amount of poured molten metal are adjusted such that ratio of slab heat amount (Q1) computed by specific relation (I) and casting mold heat amount (Q2) computed by specific relation (II) is 4.7 or less.    USE - Manufacture of copper-gallium alloy slab used for sputtering target for thin film formation of light absorption layer for thin-film solar cell.    ADVANTAGE - The method provides large-sized copper-gallium alloy slab with suppressed generation of crack.    DETAILED DESCRIPTION - Manufacture of copper-gallium alloy slab containing copper-gallium alloy by melt casting involves preparing copper and gallium in crucible, heating crucible at predetermined casting temperature under reduced pressure, obtaining molten metal of copper-gallium alloy in crucible, pouring predetermined amount of molten metal in crucible into casting mold, cooling and coagulating the poured molten metal in casting mold at room temperature, and molding into shape of slab. The casting temperature and amount of poured molten metal are adjusted such that ratio of slab heat amount (Q1) computed by relation: Q1 (J)=amount (g) of specific heat (J/gx K)x pouring molten metalx casting temperature (K) of copper-gallium alloy (I) with respect to casting mold heat amount (Q2) computed by relation: Q2 (J)= specific heat (J/gasteriskK) of casting moldx weight (g) of casting mold and casting mold temperature (II) is 4.7 or less. The casting mold temperature is temperature of casting mold before pouring molten metal.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of the casting apparatus used for manufacture of copper-gallium alloy slab.    Crucible (1)    Casting mold (2)    Storage tank (3)    Crucible main component (11)    Casting apparatus (100)