Long-period HM waves with 150-380 s periods were observed by Sakigake at 7¿106 km upstream from comet Halley. The wave amplitudes were as great as ~2nT peak to peak. The long-period waves were of an approximately linearly polarized transverse mode and propagated parallel to the ambient magnetic field. Dominant periods of the waves were found to be consistent with the local O+ (or H2O+) cyclotron period and to change as a function of the ambient magnetic field. The long-period HM waves are believed to be excited by a resonant instability with high-energy cometary heavy ions. The necessary cometary ion density for excitation at a distance of 7¿106 km from the comet can be inferred to be of the order of 10-3-10-4 cm-3 for 10-1-102 keV O+ (or H2O+) ions.