PURPOSE: To improve heat transfer efficiency by a method wherein either of the primary and secondary air streams passing between the laminated partition plates alternately is periodically changes its flow direction without changing at least the flow direction of the remaining air stream.

CONSTITUTION: The air streams discharged from a fan 17 to suck in the outdoor air and from a fan 18 to suck in the indoor air, both of which pass through a heat exchanger 15 formed by laminating partition plates, are changed its flow direction by the opening and closing operation of dampers 19W24 as illustrated in the accompanying figures.