A fuel supply device includes a flange, a pump unit, a supporting pillar, and a boss. The flange is attached to an opening portion of a fuel tank. The supporting pillar supports a sub tank such that the sub tank is positioned closer to and away from the flange. The boss is fixed at the flange and an end of the supporting pillar is inserted into the boss. The boss is made of a material different from that of the flange. The boss includes a stress concentration portion that is preferentially broken when a force equal to or greater than a predetermined value is applied to an end portion of the supporting pillar in an axis perpendicular direction.