A novel insertion sequence, which has been found in the sMMO gene coding for methane monooxygenase of methane-assimilating bacterium Methylococcus capsulatus NCIMB 11132 strain, and has inverted repeat sequences consisting of a sequence of the nucleotide numbers 5-19 of SEQ ID NO: 1 at the both ends, can be utilized as effective means for genetic analysis including creation of insertion mutant strains, gene mapping, promoter searching, insertion of genetic information into chromosomal DNA, disruption of specific gene and the like, or utilized for improving methane-assimilating bacteria by chromosomal genetic engineering techniques.