This invention relates to an agent for promoting differentiation of an ES cell, preferably an agent for promoting differentiation of an ES cell into a hepatocyte or a prophylactic agent for teratoma, comprising uPA. Furthermore this invention relates to a method of promoting differentiation of an ES cell, preferably a method of promoting differentiation of an ES cell into a hepatocyte, comprising the step of contacting uPA with the ES cell, or a method of preparing a hepatocyte comprising the step of contacting uPA with an ES cell to differentiate the ES cell into a hepatocyte.