To cool a blade type server disposed in an air-conditioned room, the following arrangements are made. The first is at least one shell having a ventilation passage disposed in the air-conditioned room. The second is, the following are disposed in a ventilation passage: racks, in which blade type servers each composed of a case with slim boards housed therein are stacked; cooling coils each having a coolant passage and a cooling fin and cooling a passing air; and at least one fan unit having axial-flow fans placed therein and producing air currents in one direction. The third is the fan unit forces a cooling air to flow in one direction in the ventilation passage thereby to cool the servers in the racks. The cooling coils and racks are disposed alternately so that warmed cooling air after passing through the rack is cooled by the cooling coil and then cools the next rack.