In order to express the FLP recombinase in B. mori cultured cell line, BmN-4, transient expression system using a heat shock protein gene (hsp70) promoter of Dorosophilla melnogaster was constructed. This vector was designated as pHsSV. Activity strength of the hsp70 promoter was compared with that of immediate early gene (IE-1) and polyhedrin gene of BmNPV employing the E. coli $eta$-galactosidase gene as a reporter gene. The result showed that the pHs $eta$-gal plasmid vector expressed the $eta$-galactosidase at 2nd and 3rd day after the transfer of plasmid DNA into BmN-4 cells, which was similar to that of pIE1 $eta$-gal vector, but different from that of a recombinant virus, vBm $eta$-gal. For the construction of FLP recombinase transient expression vector, the FLP recombinase gene was cloned by polymerase chain reaction technique. To express the FLP recombinase, this gene was inserted into pHsSV plasmid vector, under the control of the hsp70 promotor, and tranfected in BmN-4 cells. The expressed FLP recombinase was estimated at 44kDa on a 12.5% SDS-PAGE.