The tank attachment structure includes a floor panel (2) having a raised portion (11) and an opening (12) protruding toward a passenger compartment, a tank (4) disposed inside the raised portion (11), a plurality of upper bands (5) configured to cover the tank (4) from above, a lower plate (6) connected to the upper bands (5), and a pair of cross members (3) provided in front of and behind the tank (4) and extending in the vehicle width direction. The lower plate (6) has a width dimension greater than that of the upper bands (5) in a vehicle width direction and is provided by the number smaller than that of the upper bands (5) to cover the opening (12). A front end portion (26) and a rear end portion (28) of the lower plate (6) are attached to the pair of cross members (3), respectively.