A method for producing human pluripotent stem cells with enhanced differentiation efficacies, comprising: introducing at least one molecule selected from the group consisting of (a) to (c) into human pluripotent stem cells (PSCs) or human somatic cells which are to be induced toward human PSCs; (a) human TET1 protein, (b) a nucleic acid that encodes human TET1 protein, and (c) a vector into which a nucleic acid that encodes human TET1 protein has been inserted, wherein said human TET1 protein is a mutant human TET1 protein which has enhanced stability as compared to a wild type human TET1 protein, and in which the second amino acid from the N-terminus of said mutant human TET1 protein differs from that of the wild-type human TET1 protein, wherein said mutant human TET1 protein is selected from the group consisting of: (i) a mutant human TET1 protein in which the second amino acid from the N-terminus is substituted with another amino acid in the amino acid sequence of SEQ ID NO: 2, 4 or 6 or the N-terminal portion thereof comprising the DNA-binding domain encompassing the lysine 585 to lysine 613; and (ii) a mutant human TET1 protein in which one to fifty amino acids have been inserted between the first and the second amino acids of the N-terminus of the wild type human TET1 protein of the amino acid sequence of SEQ ID NO: 2, 4 or 6 or the N-terminal portion thereof comprising the DNA-binding domain encompassing the lysine 585 to lysine 613. introducing at least one molecule selected from the group consisting of (a) to (c) into human pluripotent stem cells (PSCs) or human somatic cells which are to be induced toward human PSCs; (a) human TET1 protein, (b) a nucleic acid that encodes human TET1 protein, and (c) a vector into which a nucleic acid that encodes human TET1 protein has been inserted, (a) human TET1 protein, (b) a nucleic acid that encodes human TET1 protein, and (c) a vector into which a nucleic acid that encodes human TET1 protein has been inserted, wherein said human TET1 protein is a mutant human TET1 protein which has enhanced stability as compared to a wild type human TET1 protein, and in which the second amino acid from the N-terminus of said mutant human TET1 protein differs from that of the wild-type human TET1 protein, wherein said mutant human TET1 protein is selected from the group consisting of: (i) a mutant human TET1 protein in which the second amino acid from the N-terminus is substituted with another amino acid in the amino acid sequence of SEQ ID NO: 2, 4 or 6 or the N-terminal portion thereof comprising the DNA-binding domain encompassing the lysine 585 to lysine 613; and (ii) a mutant human TET1 protein in which one to fifty amino acids have been inserted between the first and the second amino acids of the N-terminus of the wild type human TET1 protein of the amino acid sequence of SEQ ID NO: 2, 4 or 6 or the N-terminal portion thereof comprising the DNA-binding domain encompassing the lysine 585 to lysine 613. (i) a mutant human TET1 protein in which the second amino acid from the N-terminus is substituted with another amino acid in the amino acid sequence of SEQ ID NO: 2, 4 or 6 or the N-terminal portion thereof comprising the DNA-binding domain encompassing the lysine 585 to lysine 613; and (ii) a mutant human TET1 protein in which one to fifty amino acids have been inserted between the first and the second amino acids of the N-terminus of the wild type human TET1 protein of the amino acid sequence of SEQ ID NO: 2, 4 or 6 or the N-terminal portion thereof comprising the DNA-binding domain encompassing the lysine 585 to lysine 613.