a closer that closes the tank opening with air-tight sealing and has a cap engagement element engaging with the opening engagement element via rotation of the closer by a preset angle;

a handle mechanism that is mounted on an upper portion of the closer in a rotatable manner to rotate the closer in a closing direction and an opening direction of the tank opening; and

a torque mechanism that is interposed between the handle mechanism and the closer, the torque mechanism having (i) a first engagement element, and (ii) a second engagement element engaging with the first engagement element, the torque mechanism being configured to transmit a rotational torque applied to the handle mechanism to the closer via engagement of the first engagement element with the second engagement element, wherein

the second engagement element includes an arm support member and a torque arm protruded from the arm support member, the torque arm having (i) an arm main body that is protruded from the arm support member and is structured as a cantilever beam with a free end arranged to face the arm support member across a preset gap, and (ii) a torque engagement projection that is protruded from the arm main body to engage with the first engagement element,

the torque arm being configured to be deformed in a direction perpendicular to a longitudinal axis of the torque arm at a two-point support where the free end is in contact with the arm support member, and to transmit the rotational torque in the closing direction applied from the handle mechanism to the closer, before the engagement of the first engagement element with the torque engagement projection is released not to transmit the rotational torque to the closer when the rotational torque exceeds a predetermined level,

the torque mechanism has a torque plate held between the handle mechanism and the closer in a freely rotatable manner, and the torque arm is protruded from an outer circumference of the torque plate,

the torque mechanism further comprises a third engagement element that is a torque arm that is constructed as a cantilever beam, and engages with the first engagement element to transmit a greater rotational torque in the opening direction than the engagement of the first engagement element with the second engagement element in response to a rotation of the handle mechanism in the opening direction, and

the third engagement element has an engagement end at an end of the cantilever beam, the torque arm of the third engagement element being inclined at a one-point support of a base of the torque arm under application of pressure from the first engagement element to the engagement end.