The invention provides a method of improving the efficiency of establishment of induced pluripotent stem cells by increasing, in a nuclear reprogramming step of somatic cell, the level of activated form of one or more proteins selected from the group consisting of Ras family members, PI3 kinase, RalGEF, Raf, AKT family members, Rheb, TCL1 and S6K. The invention also provides a method of producing induced pluripotent stem cells by contacting a somatic cell with a nuclear reprogramming substance and one or more of such proteins and nucleic acids that encode such proteins. The invention further provides an induced pluripotent stem cell that has an exogenous nucleic acid encoding such a protein, as well as agents for use in the aforesaid methods.