An adsorbent canister includes a casing defining a first adsorption chamber, a second adsorption chamber and a diffusion chamber therein, and an adsorbent capable of adsorbing the fuel vapor and filled in the first adsorption chamber and the second adsorption chamber. The first adsorption chamber and the second adsorption chamber communicate with each other via the diffusion chamber. The casing has on a side wall thereof a fuel introducing port configured to introduce fuel vapor into the first adsorption chamber and an air communicating port configured to introduce air into the second adsorption chamber. The diffusion chamber has at least one of a first diffusion chamber and a second diffusion chamber. The first adsorption chamber is positioned above the first adsorption chamber and extends over the entire length of the first adsorption chamber. The second diffusion chamber is positioned below the second adsorption chamber and extends over the entire length of the second adsorption chamber.