A fuel vapor emission control system for connection to an engine having a fuel tank, so as to purge fuel vapor introduced thereto into the engine, comprises primary and secondary adsorbing layers for temporarily adsorbing fuel vapor introduced thereto. The primary and the secondary layers are connected in series. The air is introduced into, in turn, the primary and the secondary layers so that the air passing therethrough desorbs fuel vapor therefrom, and then into the engine, together with fuel vapor desorbed from the primary and the secondary layers, when the engine is running. During an engine fuelling operation, fuel vapor is introduced from the fuel tank to the primary layer. When the engine is running, fuel vapor is introduced, from the fuel tank, into the engine by bypassing the primary and the secondary layers.