A compound which captures a multisite phosphorylated peptide or protein specifically to a phosphorylation site and a method for detecting the peptide or protein using the compound. Particularly, a compound which specifically detects an excessively phosphorylated tau protein observed in the brain affected by Alzheimer's disease and a method for diagnosing Alzheimer's disease in vitro or in vivo using the compound are provided. By bringing a metal complex compound having two dipicolylamine (Dpa) moieties and a spacer including a chromogenic or luminescent functional or atom group into contact with a multisite phosphorylated peptide or protein, the compound recognizes the distance between phosphate groups and specifically binds to the peptide or protein, and a multisite phosphorylated peptide or protein or kinase activity is optically detected by measuring the change, or a multisite phosphorylated peptide or protein or kinase activity is imaged by an optical imaging method applying the change in luminescence.