Magnetic properties of YBa 2 Cu 3 O 7−Δy was investigated by dc- and ac-magnetic susceptibility as a function of oxygen deficiency Δy. The susceptibility in the normal state at low temperatures is dominated by the Curie-Weiss law, while the susceptibility at high temperatures deviates from the Curie-Weiss law. The deviations were successfully explained by assuming the triplet spin-pair excitation from the antiferromagnetically coupled Cu 2+ ions in the ground state.