Abstract: Nitroxyl compounds represented by general formula (I) which are usable in acquiring information about active oxygen or in vivo free radicals as biological images such as magnetic resonance images or magnetoencephalograms, wherein A represents hydrogen or —L—(CH2)n—OCOR (wherein L represents —COO— or -alkylene-COO—; and R represents C1-4 alkyl); R1, R2, R3, and R4 represent each C1-4 alkyl; and n is a number of from 1 to 4. Drugs and reagents containing the above compounds are usable in preventing, treating or diagnosing ischemic diseases, digestive diseases, cancer, cranial nervous diseases accompanied by nerve degeneration, inflammation, cataracts, or drug-induced organopathy.