A new thermostable luciferase of the following mutant luciferase (a) or (b): (a) a mutant of a wild-type luciferase comprising the amino acid sequence of SEQ ID NO: 1, wherein phenylalanine at position 292 and/or phenylalanine at position 294 in the amino acid sequence of SEQ ID NO: 1 is substituted with another amino acid; or (b) a mutant of a luciferase having 93% or more homology with the amino acid sequence of SEQ ID NO: 1, wherein in the amino acid sequence of the mutant, the amino acid at a site corresponding to position 292 and/or position 294 in the amino acid sequence of SEQ ID NO: 1 is substituted with another amino acid.