An object of the present invention is to provide a novel drug for treating cancer. The present invention relates to a pharmaceutical composition for treating or preventing cancer which comprises, as an active ingredient, a triethylene glycol of the following formula (I) or a derivative thereof:wherein R1 and R2 are independently selected from hydrogen, C1-6 alkyl, C1-6 haloalkyl, or —C(═O)R3 and R3 is selected from C1-6 alkyl or C1-6 haloalkyl.
