One of the purposes of the present invention is to provide a heterocyclic derivative that has an IAP (particularly XIAP) binding (inhibiting) activity. Another of the purposes of the present invention is to provide a heterocyclic derivative that has an IAP (particularly XIAP) binding (inhibiting) activity and exhibits a protein degradation induction activity. The present invention provides a compound represented by formula (I) (the symbols in the formula are as defined in the present Description) and salts thereof.