Calcium-binding photoproteins showing the luminescence pattern with a slow decay of are desired. The invention provides a mutant apoprotein comprising an amino acid sequence wherein the 23rd to 34th amino acids in the amino acid sequence of SEQ ID NO: 2 are substituted with an amino acid represented by formula I below: Xaa23-Xaa24-Xaa25-Xaa26-Xaa27-Xaa28-Xaa29-Xaa30-Xaa31-Xaa32-Xaa33-Xaa 34; having a function to bind to the peroxide of coelenterazine or the peroxide of a coelenterazine analogue to form a photoprotein capable of emitting light under the action of calcium ions; and, having a half decay time of the luminescence emitted by binding of the photoprotein to calcium ions being not less than 2 seconds.