To obtain a highly sensitive, stable, inexpensive and analyzable composition useful for electrochemical luminescent labels by compounding plural chemical species which can emit electromagnetic radiations having different wavelengths, respectively, such as an organic Ru compound or an organic Os compound.

This composition comprises the first chemical species such as a Ru-containing organic metal or Os-containing organic metal compound of the formula [M is ruthenium, osmium; P is the multidentate of M; L1-L6 are each the ligand of M; B is a biological substance, synthetic substance or non-biological substance covalently bound to one or more of P, L1-L6 through one or more amide bonds or amine bonds; the biological substance and the synthetic substance can compete with the biological substance on competitive binding reactions with complementary substances; (m) is an integer of ≥1; (n), (o), (p), (q), (r), (s) are each 0, an integer; (t), (u) are each an integer of ≥1], and further one or more chemical species. The chemical species can emit electromagnetic radiations having different wavelengths, respectively.