The present invention provides an expression vector which is effective in an efficient establishment of transformed cells which express the aimed protein gene in a high level. An expression vector which has a cassette for expressing the drug selective marker gene containing mRNA destabilizing sequence, at least one element for stabilizing the gene expression and a cassette for expressing the gene of the aimed protein. Preferably, the mRNA destabilizing sequence is derived from AT-rich sequence existing in the 3′-untranslated region of cytokine, interleukin or proto-oncogene, and the element for stabilizing the gene expression is derived from Chinese hamster genome.