Supply air conduit (7) along which air is sent from outdoors to indoors by the air supply fan (9), exhaust air conduit (8) along which air is sent from indoors to outdoors by the air exhaust fan (10), and heat exchange element (6) provided at a position where supply air conduit (7) and exhaust air conduit (8) intersect, for exchanging heat between indoor air and outdoor air for ventilation are included. In addition, in exhaust air conduit (8), humidity detection unit (14) is provided at a position upstream of heat exchange element (6), and, in supply air conduit (7), temperature detection unit (13) is provided at a position upstream of heat exchange element (6). In addition, controller (11) reduces, when a temperature detected by temperature detection unit (13) and humidity detected by humidity detection unit (14) are predetermined values, speed of an air supply motor and an air exhaust motor so that air flow rates at which moisture neither condenses nor freezes are attained.