A fuel vapor treatment device for suppressing emission of fuel vapor from a fuel tank or the like of an automotive vehicle. The fuel vapor treatment device comprises a casing having a charge port connected to a fuel tank, a purge port connected to an intake section of an engine, and an atmospheric air port through which atmospheric air is introduced. Fuel vapor adsorbing material is filled in the casing. Additionally, an adsorbing material cartridge is disposed in a part of the casing and includes a cylindrical cartridge main body section having a cross-sectional area smaller than that of the casing. Fuel vapor adsorbing material is filled in the cartridge main body section. Air introduced from the atmospheric air port is flowable through the inside of the cartridge main body section to the fuel vapor adsorbing material in the casing.