Provided are: an expression vector for secreting a protein (Z) to be recovered or a fusion protein having the protein (Z) moiety therein; a method for producing a transformant using the expression vector; the transformant; and a method for producing a protein using the transformant. An expression vector comprising an expression cassette containing a structural gene sequence (y) encoding a protein (Y), a structural gene sequence (z) located downstream from the structural gene sequence (y) and encoding a protein (Z) that is a protein to be recovered, and a promoter sequence and a terminator sequence for expressing a fusion protein containing the protein (Y) moiety and the protein (Z) moiety, characterized in that the protein (Y) is a full-length protein of protein disulfide isomerase 1 (PDI1), a partial protein of PDI1, or a mutant protein of the full-length protein or the partial protein.