To provide a filler cap that inexpensively treats and eliminates harmful gas evaporated from fuel in a fuel tank without releasing it into the atmosphere.

The filler cap includes a cap body 10 that is provided with a pipe 70 communicating with a carburetor of an engine, and an air path 26 that is formed in the cap body 10 and is communicating with the fuel tank and also with the carburetor via the pipe 70. The filler cap also includes an adsorption chamber 77 that is formed in the cap body 10 and is communicating with the air path 26 via an internal communicating path 83, and an adsorbing member 82 that is provided in the adsorption chamber 77 and has a function of adsorbing and releasing gas generated in the fuel tank.