A canister is provided with a flange for partitioning part of one axial end of an adsorption material chamber and having a first cylindrical portion protruding toward the adsorption material chamber for defining a space communicating with a purge port, a second cylindrical portion extending toward a first axial end portion of a case for defining a diffusion chamber communicating with a charge port, and orifices for communicating the adsorption material chamber with the diffusion chamber. Also provided are a first partition wall positioned close to a top end of the first cylindrical portion and having an annularly-shaped orifice, and a second partition wall axially spaced apart from the first partition wall toward a second axial end portion and having communication holes. These partition walls cooperate with each other such that a second diffusion chamber not charged with an adsorption material is defined between them, for optimal charge-gas conversion-and-diffusion.