A cooling device for an under-floor device for a vehicle is provided with: a base plate having an under-floor device mounted on the rear face side thereof; a heat radiation section mounted to the front face side of the base plate and radiating heat which is transferred from the under-floor device through the base plate; a cover for covering the heat radiation section and having side face openings in both side faces of the cover which face the movement direction of a vehicle, the side face openings enabling relative air flow to flow therein and enabling the flowed-in relative air to flow therefrom; and guide plates provided in the side gap regions between the heat radiation section and the side face of the cover, the side face being located on the side into which the relative air flow flows, the guide plates closing at least a part of the gap region between the heat radiation section and the faces of the cover which connect both side faces of the cover, the guide plate conducting to the heat radiation section the relative air flow which flows in from the side face openings.