The present invention provides a mutant beetle luciferase and the like, having mutation in which the amino acid corresponding to valine at position 288 in the amino acid sequence of wild-type Photinus pyralis luciferase is isoleucine, leucine or phenylalanine, mutation in which the amino acid corresponding to leucine at position 376 in the aforementioned sequence is proline, mutation in which the amino acid corresponding to glutamic acid at position 455 in the aforementioned sequence is valine, alanine, serine, leucine, isoleucine or phenylalanine, or mutation in which the amino acid corresponding to glutamic acid at position 488 in the aforementioned sequence is valine, alanine, serine, leucine, isoleucine or phenylalanine, in the amino acid sequence encoding a wild-type beetle luciferase, and characterized in that a luminescence intensity due to a luciferin-luciferase luminescence reaction in a 0.9% by mass NaCl solution is 50% or more of that in a NaCl-free solution.