The present invention provides an apoptosis-inducing agent capable of inducing apoptosis in various cells including tumor cells and other cells in a simple manner, etc. The apoptosis-inducing agent of the present invention comprises a peptide shown in (a) or (b) below, a derivative thereof, or a salt of the peptide or the derivative: (a) a peptide which comprises an amino acid sequence represented by the formula: C-X-D-X-X-X-X-Y-X-C-X-C (I) or (b) a peptide which comprises an amino acid sequence with deletion, substitution or addition of one or several amino acids in the amino acid sequence represented by formula (I) and which has apoptosis-inducing activity.
