To clarify the route and source of Enterobacter sakazakii infection in a basic study, we analyzed powder infant formula milk (PIF), which may be an important source of infantile infection, regarding contamination with Enterobacteriaceae including this type of bacteria, and conducted drug sensitivity tests with various antimicrobial agents. Enterobacteriaceae was isolated 36 (24.2%) of 149 PIF samples. These comprised of 12 (19.7%) of 61 domestically produced samples and 24 (27.3%) of 88 imported samples. E. sakazakii was isolated in 9 (6.6%) of the 149 PIF samples. These comprised 4 (6.6%) of 61 domestically produced samples and 5 (5.7%) of 88 imported samples. In 8 of the 9 samples in which E. sakazakii was isolated, the bacterial levels were estimated to be 0.36 MPN/100 g. However, one imported sample showed a bacterial level of 0.91 MPN/100 g. In the drug sensitivity tests of E. sakazakii isolated from PIF, we compared the MIC(90) values. E. sakazakii was highly sensitive to 9 agents: cefotaxime, ceftriaxone, cefoperazone, ceftazidime, cefpirome, cefozopran, gentamicin, meropenem, and ciprofloxacin, and moderately sensitive to 5 agents: piperacillin, erythromycin, minocycline, chloramphenicol, and rifampicin. However, it was resistant to 2 agents, ampicillin and lincomycin.