Abstract: According to the present invention, a method for treating malignant mesothelioma using REIC/Dkk-3 and a therapeutic agent for malignant mesothelioma comprising REIC/Dkk-3 are provided. A mesothelioma therapeutic agent is provided, comprising the following REIC/Dkk-3 DNA or a vector comprising the DNA as an active ingredient: (a) a DNA consisting of the nucleotide sequence shown in SEQ ID NO: 1; or (b) a DNA hybridizing under stringent conditions to a DNA consisting of a nucleotide sequence complementary to the nucleotide sequence shown in SEQ ID NO: 1 and encoding a protein having apoptosis-inducing activity and IL-7 production-accelerating activity in cells.