PURPOSE: To provide ventilating function in addition to the function of the bow window and prevent invasion of rain water into a room when it is rainy by providing a fan at the inside of a box-type accommodating chamber formed by the ceiling plate as well as the roofing plate of the bow window and the outer wall of a house.

CONSTITUTION: The fan 7 is provided at the inside of the accommodating chamber 6 formed between the ceiling plate 4 as well as the roofing plate 3 of the bow window and the outer wall 109 of the house while the indoor side surface and the outdoor surface of the ceiling plate 4 and a sash 2 are provided with a through port 8 for suction and the through port 9 for discharge as well as a venting port 10 for suction and the venting port 11 for discharge. The through port 12 for supplying air and the venting port 13 for supplying air are provided at the indoor side surface of a base plate 5 and the sash 2 whereby the functions of forced air discharging and natural air supplying may be obtained in addition to the function of bow window when the fan 7 is driven. The through port 9 for discharge and the venting port 11 for discharge are provided horizontally and are covered by the roofing plate 3, therefore, the invasion of rain water may be prevented when it is rainy without providing any eaves or hood additionally.