NOVELTY - A compound semiconductor solar cell consists of subcell (201) containing compound semiconductor layer (40), subcell (204) containing compound semiconductor layer (II), subcell (206) containing compound semiconductor layer (III), and a grading buffer layer (202) containing compound semiconductor layer (14). The lattice constant difference between the layer which adjoins subcell (204) most among the layers of grading buffer layer, and layer (II) is larger than the lattice constant difference of two adjacent interlayers of the grading buffer layer.    USE - Compound semiconductor solar cell (claimed).    ADVANTAGE - The compound semiconductor solar cell has excellent characteristics.    DETAILED DESCRIPTION - A compound semiconductor solar cell consists of subcell (201) containing compound semiconductor layer (40), subcell (204) containing compound semiconductor layer (II), provided on subcell (201), subcell (206) containing compound semiconductor layer (III), provided on subcell (204), and a grading buffer layer (202) containing compound semiconductor layer (14), provided between subcells (204,201). The lattice constant of layer (II) is smaller than that of layers (40,14). The lattice mismatch rate of lattice constant of layer (40) with respect to the lattice constant of layer (II) is 0.1% or more. The lattice mismatch rate of lattice constant of layer (14) with respect to the lattice constant of layer (II) is 0.1% or more. The lattice constant difference between the layer which adjoins subcell (204) most among the layers of grading buffer layer, and layer (II) is larger than the lattice constant difference of two adjacent interlayers of the grading buffer layer. The thickness of layer which adjoins subcell (201) most among the layer of grading buffer layer is 1.5-2 mu m.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the compound semiconductor solar cell.    Compound semiconductor layers (14,40)    Contact layer (43)    Metal layer (50)    Support substrate (51)    Subcells (201,204,206)