The fim inversion system of Escherichia coli (E. coli) can behave as a unidirectional switch in an efficient manner. We have developed a new expression system for E. coli, comprising the arabinose-inducible fimE gene and the fim invertible DNA segment containing a constitutively active promoter. In this system, the target gene is cloned with the promoter in the OFF orientation, resulting in no transcribed product. When induced by arabinose, the active promoter is switched to the ON orientation via FimE-catalyzed DNA inversion, and the gene is expressed. Our expression system exhibited very tightly controlled basal expression and high induced expression, with simple induction by inexpensive arabinose. These characteristics make our system suitable for large-scale expression or for production of toxic proteins.