A use of vivapain-4 (VX-4), which is a cysteine protease of Plasmodium vivax, showing pH-dependent switching of substrate specificity, is provided. More specifically, a method of treating a parasitic disease caused by Plasmodium vivax by inhibiting VX-4; a method of screening a protease inhibitor acting on VX-4, wherein the protease inhibitor is useful as an anti-malarial agent acting on Plasmodium species, for example, Plasmodium vivax; and a method of identifying the activity of VX-4, are provided.