There is provided a cooler for heater-containing box in which an outside air fan for carrying air on an outside air path and an inside air fan for carrying air on an inside air path are arranged such that a rotating shaft of each fan is in parallel with an air intake direction, and a heat exchanger for exchanging sensible heat of outside air and inside air is arranged between the outside air path and the inside air path such that a suction port at a longitudinal air-path side of the heat exchanger is aligned with a blowout port of the fan. The cooler for heater-containing box capable of making the depth thinner can be provided.