This invention is to provide a photoprotein which binds with a ligand specific for a substance to be detected at a binding ratio of 1:1 such that the luminescence activity is not reduced by binding with the ligand, a conjugate comprising the luminescent photoprotein and ligand, and a substance detection method which employs the conjugate as a marker. A calcium-binding photoprotein is produced having cysteine residue introduced within the 4th amino acid residue from the amino-terminus. A conjugate is formed by binding a ligand specific for a substance to be detected to the calcium-binding photoprotein, in a binding ratio of 1:1, via the introduced cysteine residue. The conjugate may be utilized as a marker for a substance to be detected.