Abstract Several studies have reported benefits derived from cancer treatment using various heavy-ion beams. Based on these reports, the National Institutes for Quantum and Radiological Science and Technology started developing intensity-modulated composite particle therapy (IMPACT) using He-, C-, O-, and Ne-ions. In ion beam therapy, nuclear interactions in the beamline devices or patient produce secondary neutrons. This study evaluated the characteristics of secondary neutrons in IMPACT. Neutron ambient dose equivalents were measured using WENDI-II. Measurements were performed under realistic case scenarios using He-, C-, O- and Ne-ion beams. Moreover, neutron ambient dose equivalents generated by He-, C-, O- and Ne-ion beams were compared with neutron ambient dose equivalents in proton therapy. No differences exist in the distance-dependence even when the primary ions are different. Neutrons generated by primary ion beams of high atomic numbers tend to emit forward. Moreover, in contrast with proton therapy, IMPACT can reduce neutron doses.