In the bi-nuclear metal complex of the first aspect of the present invention, M is an element of the periodic table 2A (namely, an alkali earth metal such as Be, Mg, Ca, Sr, Ba, and Ra) or an element of the table 2B (namely, Cd and Hg) or Zn; L1, L2 and L3 each are a ligand derived from a compound having a hydroxyl group and an aromatic nitrogen atom (N) within the molecule and being represented b