Many antibiotic resistance genes found in gram-negative clinical isolates are a part of a gene cassette inserted into an integron. Incidence of the integron carriage in extended-spectrum $eta$-lactamases (ESBLs)-producing clinical isolates is considerably high. In this study, the presence of class 1 integron was investigated among ESBL-producing K. pneumoniae, and the genetic location of the integron and $bla_{ESBL}$ was determined to elucidate the genetic linkage of the integron and $bla_{ESBL}$ gene. Thirty-nine clinical isolates of ESBL-producing K. pneumoniae and 36 transconjugants were used. The presence of class 1 integron was examined by PCR and Southern hybridization, and the location of ESBL gene and integron was studied by Southern hybridization with the probes for $bla_{TEM},;bla_{SHV},;bla_{CMY-1}$, and integron. PCR and Southern blot hybridization studies showed that 31 of 39 (79.5%) clinical isolates and 21 of 36 (58.3%) transconjugants carried integrons. Furthermore, integron was found to be on the same conjugative plasmid on which $bla_{SHV-ESBL}$ or $bla_{CMY-1}$ gene was located, suggesting high genetic linkage of integron and $bla_{ESBL}$ genes. A sequencing study showed that PCR-amplified inserted gene cassettes contained aminoglycoside resistance genes (aadA2, aadA5, aacA4, aadB) as well as catB5, dfrA12, dfrA17, OXA-1, and OXA-2 variant. Notably, aacA4, which had been rarely found in previous studies, was the most predominant gene cassette. In conclusion, the coincidence of an ESBL gene and class 1 integron/gene cassette on the same conjugative plasmid has potential implications for the spread of ESBL-mediated drug resistance and may be responsible for the multi-drug resistance in these isolates.