The present invention aims to express influenza virus RNA polymerase on a large scale, to crystallize the influenza virus RNA polymerase, and to provide a method for screening a substance capable of serving as an active ingredient in anti-influenza drugs which target a protein highly conserved among influenza virus species.The present invention provides a complex comprising a polypeptide consisting of an amino acid sequence at positions 239-716 of the RNA polymerase PA subunit in influenza A/Puerto Rico/8/1934 H1N1 and a polypeptide consisting of an amino acid sequence at positions 1-81 of the RNA polymerase PB1 subunit in influenza A/Puerto Rico/8/1934 H1N1, as well as a method for screening a substance capable of serving as an active ingredient in anti-influenza drugs, which comprises the step of selecting a substance which inhibits the interaction between α-subunit and β-subunit 1, each constituting influenza virus RNA polymerase, in the presence of a candidate substance.