The contribution of the beam–beam (b–b) Coulomb collision effect on the fast ion slowing-down process is investigated. The effect is evaluated experimentally in the large helical device (LHD) from the response of the neutron emission rate to the direction of the tangential hydrogen beam, which is used with the tangential deuterium beam. In addition, to analyze the experimental results, a Fokker–Planck (F–P) code is improved. It is observed that the decay time of the neutron emission rate after the deuterium beam is turned off depends on the direction of the hydrogen beam. This trend can be explained by the b–b Coulomb collision effect. The hydrogen beam, which has the same direction as the deuterium beam, deforms the fast deuteron velocity distribution due to the b–b Coulomb collision. As a result, the neutron decay time becomes longer than that with the opposite direction hydrogen beam. Our F–P simulation shows that the b–b Coulomb collision effect contributes to the decay time of the neutron emission rate. This simulation result is qualitatively similar to the experimental result. For quantitative analysis, consideration of the fast ion spatial transport, which is neglected in the present simulation, is required.