To surely set the rotational torque of the torque mechanism 80, for sealing the pressure of a fuel cap 10 set by the torque mechanism 80.

The torque mechanism 80 transmits the rotational torque to a casing body 20 by the engagement between an operation section side engaging part 46 and a first torque arm 94 which is provided on a torque plate 90, when a lid body 40 is rotated in the closing direction, disengages the engagement when a specified rotational torque is exceeded, to obtain a feeling of moderation. The first torque arm 94 is formed of a cantilever extension, having a free end 94d with an interval with respect to an arm supporting section 91a. When pressed by the operation section side engaging part 46, the first torque arm 94 goes into a two-point support state, wherein the free end 94d comes a state abutted against the arm support section 91a, and the rotational torque in the closing direction is transmitted to the casing body 20, while the torque arm 94 is warped in the longitudinal direction and the rectangular direction.