A trap canister is configured to capture fuel vapor contained in breakthrough gas discharged from an adsorbent canister. The trap canister has a housing defining an adsorption chamber therein, a breathable partition member disposed in the housing and dividing the adsorption chamber into a first chamber and a second chamber, an adsorbent filled in the first chamber and the second chamber, and a latent heal storage material tilled in the first chamber in a mixed manner with the adsorbent. The first chamber is configured to receive breakthrough gas, while the second chamber is configured to communicate with the atmosphere. The first chamber has a cross-sectional area larger than that of the second chamber.