A new Haldane gap compound, Ni(dmpn) 2 N 3 (ClO 4 ) (dmpn=2,2-dimethyl-1,3-propanediamine) has been synthesized, and its structure and magnetic properties have been investigated. It has an azido-bridged linear-chain structure where each chain is separated to each other by counterions ClO 4 − . The Haldane gap and J value of this compound are estimated to be 21.6 K and −70.6 K, respectively. These absolute values are larger than those of Ni(en) 2 (NO 2 )(ClO 4 ) (hereafter abbreviated as NENP) which has a NO 2 − -bridged structure and has been extensively investigated so far.