PROBLEM TO BE SOLVED : To provide a means whereby, after a pluripotent stem cell-derived cell or its structure is transplanted, it is possible to substantially prevent a tumor from occurring in the cell or its structure. SOLUTION : An embodiment relates to a teratoma formation inhibitor that contains a compound having lysine specific demethylase 1-selective inhibitor (LSD1) inhibitory activity as an active ingredient. Another embodiment relates to a pharmaceutical or a pharmaceutical composition for use in the inhibition of teratoma formation, which contains a compound having lysine specific demethylase 1-selective inhibitor (LSD1) inhibitory activity as an active ingredient. SELECTED DRAWING : None COPYRIGHT : (C)2019, JPO&INPIT