PURPOSE: To introduce the warm air in a loft space, to exhaust the cold air in the loft space to the outside of a building through snow-melting operation and to obtain efficient snow-melting operation through heat of an outside temperature to promote snow-melting efficiency.

CONSTITUTION: An air-intake duct 7 having a plurality of outlets is provided to a loft space 4, and when an outside temperature is in excess of a set temperature, it is detected with a sensor 9, a blowing mechanism 8 is interlocked therewith to blow off the air of the outside of the building from the outlets 7a through the air-intake duct 7, and the outside air is introduced into the loft space 4. In addition, the blowing mechanism 8 is interlocked therewith to draw the air in the loft space 4 from a plurality of inlet ports 10a, and the air in the loft space 4 is exhausted to the outside of the building through an air exhaust duct 10.