A canister for trapping a fuel vapor vaporized in a fuel tank has a casing defining an adsorption chamber therein, an adsorbent capable of adsorbing the fuel vapor and filled in the adsorption chamber and a flow regulation plate disposed in the casing. The casing has a fuel introducing port configured to introduce the fuel vapor from the fuel tank into the adsorption chamber, and an air communicating port communicating the adsorption chamber with the atmosphere. The air communicating port is formed on a side surface of the casing extending in a direction of gravitational force. The flow regulation plate disposed at an end of the adsorption chamber near the air communicating port and has an opening configured to communicate the adsorption chamber with the air communicating port. The opening is positioned above the air communicating port in the direction of gravitational force.