a closer that closes the tank opening wit air-tight sealing and has a cap engagement element that engages with the opening engagement element by rotation of the closer at a predetermined angle;

a handle mechanism that is mounted on the closer in a rotatable manner and rotates the closer in either of a closing direction and an opening direction to close and open the tank opening wherein the handle mechanism includes a cover that surrounds an upper portion of the closer and a handle that is formed on the cover; and

a torque mechanism having a torque plate that is located between the handle mechanism and the closer and selectively transmits a rotational torque in the closing direction and the opening direction to the closer, wherein the torque mechanism includes:

a closing direction transmission mechanism baying a first engagement element on the closer and a second engagement element that engages with the first engagement element, the closing direction transmission mechanism transmitting a rotational torque applied to the handle mechanism in the closing direction to the closer through engagement of the first engagement element with the second engagement element and making the first engagement element released from the second engagement element when the rotational torque exceeds a preset level, wherein

the second engagement element is a torque arm that is formed as an arch-shaped shaped cantilever having a support base formed from the torque plate and a torque engagement projection,

the torque arm is supported by two points, which are located respectively at a support base and a free end of the torque arm, and

the torque arm is bent in a direction perpendicular to a longitudinal direction of the cantilever when the first engagement element presses the torque engagement projection, and

an opening direction transmission mechanism having a guide mechanism that guides the torque engagement projection to reduce a lap of the engagement of the first engagement element with the torque engagement projection when the handle mechanism rotates in the opening direction, wherein the guide mechanism includes (i) a guide projection that is formed on an end of the torque arm, and (ii) a guide groove that is formed on a bottom face of the cover to guide the guide projection in response to rotational force applied to the handle mechanism so that the torque engagement projection is led in a direction away from the first engagement element, thereby reducing the lap.