To provide a filler cap device having a tether which does not obstruct the operation when a filler port is opened and closed.

A filler tube 1 is provided with a projection 13 to be engaged, and an engaging member 5 capable of being engaged with and disengaged from the projection 13 is installed on a filler cap 2 without penetrating the part including a seal member 23 and a pressure adjusting valve 4 and partitioning inside a fuel tank from its outside in the condition that the oil supply hole 11 is closed, and the engaging member 5 is engaged with the projection 13 in the condition that the insertion part 22 of the filler cap 2 is inserted into the filler port 11 to seal it, in which disengagement can be generated by operating a push-button part 47 provided on the head 21 of the filler cap 2. This allows mounting and removal of the filler gap 2 on/from the oil supply hole 11 without turning the filler cap 2, and the tether 3 to tie the filler cap 2 with the vehicle body does not obstruct the operation when the oil supply hole 11 is opened and closed.