Abstract: An object of the present invention is to provide a novel means for finding a passive substance to the action of a selected trigger protein in a target cell system. In particular, the present invention succeeded in establishing a novel method for screening an indicator substance by the steps of contacting the trigger protein with a target cell extract to initiate the action on an unspecified indicator substance by the trigger protein, and specifying the substance changed by the action induced by the trigger protein. The primary part of the present invention comprises the following steps: 1) contacting a trigger protein prepared by a cell-free protein synthesis means with a target cell extract which contains the indicator substance that is passively produced by an action induced by the trigger protein and desired to screen, to initiate the action by the trigger protein, and 2) specifying the substance changed by the action induced by the trigger protein.