Submillimeter wave ESR of Haldane system Y2BaNiO5 has been performed. The ESR absorption lines, which were observed in the temperature range above 15 K, are attributed to the transition between the excited triplet states. Frequency-field relations of these absorption lines are analyzed by the theory which explained those observed in NENP as the transitions between the excited triplet states near k = pi. (C) 1998 Elsevier Science B.V. All rights reserved.