The object of the invention is to produce a target protein as a soluble protein using a recombinant protein expression system. The invention provides a process for producing a target protein as a soluble protein, comprising the step of expressing a protein by using a polynucleotide containing, in order, a polynucleotide encoding a secretory signal peptide, a polynucleotide encoding a basic amino acid-rich polypeptide, and a polynucleotide encoding the target protein; and also provides an expression vector used for this purpose.