The　formation　control　problem　of　a　multi-vehicle　system　is　dealt　with　by　transforming　it　to　a　leader-following　consensus　problem.　We　focus　on　the　case　of　constant　spacing　policy,　i.e.,　to　design　a　control　strategy　to　make　all　the　vehicles　achieving　a　common　value　of　velocities　and　maintaining　a　constant　spacing　between　the　vehicles.　The　leading　vehicle　(leader)　is　assumed　to　travel　freely.　We　discuss　three　types　of　communication　topologies　between　the　vehicles.　For　each　of　them　we　propose　a　control　protocol　of　the　following　vehicles　(followers)　by　applying　a　leader-following　consensus　design　approach　and　present　a　condition　of　input-to-state　stability　(ISS)　of　the　formation.　Simulation　examples　show　that　with　these　communication　topologies,　the　designed　strategy　can　make　the　system　achieve　ISS.