The present invention provides an “in vivo and in vitro real-time bioluminescence imaging means,” which can transmit a detection signal promptly in response to an external signal, while taking advantage of a single-molecule-format luminescent probe as a bioluminescent means.The present invention is characterized by using, as a single-molecule-format luminescent probe utilizing the increase and decrease of a second messenger level as an index, a fusion protein including a single-chain protein containing a second messenger recognition protein and optionally a peptide which is capable of binding with the protein, and linked respectively to the N-terminus and the C-terminus thereto, an N-terminal fragment (N-LE) and a C-terminal fragment (C-LE) generated by dissecting a luminescent enzyme (LE). A single-molecule-format luminescent probe could be provided, which makes it possible to observe light emission (extinction) in vivo as well as in vitro, as a result of a conformational change of the second messenger recognition protein induced by the binding (unbinding) with a second messenger, and the subsequent association or dissociation between the N-LE and the C-LE flanked at both termini of the recognition protein.