To provide a complex between aequorin and a biotin-binding protein having a high light-emitting activity at such a level that can be used for a labeling substance such as a diagnostic agent, and to provide a method for simply and efficiently producing the complex.

Either a recombinant aequorin containing, as a constituent component, an apoprotein in which one cysteine is introduced into the fourth amino acid residue from the amino terminal of a specific amino acid sequence or a recombinant aequorin containing, as a constituent component, an apoprotein in which one cysteine is introduced into the sixth amino acid residue from the amino terminal of another specific amino acid sequence is bound through the cysteine to biotin-binding protein. Thereby, the complex is produced.