The present invention provides a method of improving the efficiency of establishment of induced pluripotent stem (iPS) cells by inhibiting p38 function in the step of somatic cell nuclear reprogramming. The 38 function can be inhibited by bringing an inhibitor selected from the group consisting of (1) a chemical inhibitor of p38 (2) a dominant negative mutant of p38 or a nucleic acid that encodes the same, (3) a nucleic acid selected from the group consisting of siRNAs and shRNAs targeted to p38 and DNAs that encode the same and (4) an inhibitor of p38 pathway into contact with a somatic cell and the like. The present invention also provides an agent for improving the efficiency of establishment of induced pluripotent stem cells, which contains an inhibitor of p38 function, particularly an inhibitor selected from the group consisting of (1) a chemical inhibitor of p38 (2) a dominant negative mutant of p38 or a nucleic acid that encodes the same, (3) a nucleic acid selected from the group consisting of siRNAs and shRNAs targeted to p38 and DNAs that encode the same and (4) an inhibitor of p38 pathway. Moreover, the present invention provides a production method of iPS cells, which includes bringing a nuclear reprogramming substance and an inhibitor of p38 function into contact with a somatic cell.