Abstract: A fuel supply device includes a flange unit configured to be attached to an upper wall of a fuel tank to close an opening of the fuel tank, a pump unit configured to be disposed within the fuel tank, and a coupling mechanism moveably coupling the pump unit to the flange unit in a vertically slidable manner. The coupling mechanism includes a fixed-side rail member having a rail portion extending vertically from the flange unit, and a movable-side rail member having a channel-groove shaped rail groove extending from the pump unit. The channel-groove shaped rail groove is slidably engaged with the rail portion of the fixed-side rail member. A recess having a semi-circular cross section is formed at a corner of the rail groove of the movable-side rail member.