The present invention relates to a process for preparing a foreign protein comprising culturing a bacterium containing the cysteine synthase (cysK) gene and a gene encoding the foreign protein. The present invention comprises the steps of culturing a bacterium transformed with an expression vector containing a gene encoding a serine-rich foreign protein and an expression vector containing the cysK gene, or a bacterium transformed with an expression vector containing the cysK gene and a gene encoding a serine-rich foreign protein and isolating the foreign protein therefrom. The present invention is expected to be widely used to increase the production yield of a serine-rich foreign protein.