Hard to say just one ! They have plants worldwide.
Where in Japan was Honda made?
As Honda is world recognized brand, it may have many branches, so it is difficult to tell you exact.
Honda Hamamatsu Factory - Hamamatsu, Japan
Honda Hosoe Plant - Hosoe, Japan
Honda Kikuchi Plant - Kumamoto Prefecture, Japan
Honda Japan - Suzuka, Mie, Japan
Honda Sayama Factory - Sayama, Saitama, Japan
