To provide a fuel tank having an upper tank and a lower tank formed independently, which is configured to decrease weight while sufficiently maintaining rigidity.

A fuel tank 1 is made of thermoplastic synthetic resin. The fuel tank 1 is formed by joining together an opening peripheral edge 11 of an upper tank part 10 and an opening peripheral edge 21 of a lower tank part 20, which are divided and independently subjected to injection molding. The upper tank part 10 and the lower tank part 20 are respectively formed of thermoplastic resin foamed at an expansion ration of 1.1 to 2.0.