The magnetic behavior of the nickel cubane complex [Ni-4(OMe)(4)(sal)(4)(MeOH)(4)] (Hsal = salicylaldehyde) was studied by using a Hartshorn bridge equipped with a He-3/He-4 dilution refrigerator. This complex has long been known to have a well-isolated high-spin ground state with S = 4 and it was suggested to have an easy-axis type anisotropy. AC magnetic susceptibility data clearly show a slow magnetization reversal below 0.3 K, characteristic of single-molecule magnets. The Arrhenius plot of the relaxation rate provides the activation energy of about 5 K.