A lock device of a fuel tank cap includes: a cap configured to open and close a fuel filler port; an operating unit; a fastening unit movable in conjunction with an operation of the operating unit; a lock unit configured to lock movement of the fastening unit from a fastening position to a allowing position; and a detection switch configured to detect start of the movement of the fastening unit accompanying the operation of the operating unit. The fastening unit includes: a first fastening unit of which the start of movement can be detected by the detection switch; and a second fastening unit which enables locking by the lock unit and which is integrally moved with the first fastening unit after the first fastening unit is moved by a predetermined dimension so that the second fastening unit can be moved from the fastening position to the allowing position.