To provide an opening section structure of a fuel tank detaching a tank cap without damaging positioning protrusions provided in the tank cap.

The opening section 10 has a cylindrical portion 11 extending outwardly from a tank body 1, and a metallic annular tank cap engaging member 30 having a plurality of locking holes 32 is attached to the cylindrical portion. A plurality of engaging claw parts 24 of a metallic annular attaching member 20 having a plurality of the engaging claw parts 24 to be engaged in the locking holes are engaged in the locking holes 32 to hold the tank cap 40 with the attaching member 20 and the tank cap locking member 30. The tank cap 10 has tank cap positioning projections 42, and the tank cap locking member 30 has positioning holes 37 for insertion of the tank cap positioning projections 42, and at least one of the enlarged engaging claw parts 28 of the attaching member 20 and at least one of enlarged locking holes 38 of the tank cap locking member 30 to which the enlarged engaging claw part is engaged are respectively made longer in a circumferential direction than remaining engaging claw parts 24 and remaining locking holes 32.