To realize an energy-saving and cost reduction in taking in external air, considering that a rain water permeation pipe of a rain water permeation trench of which the function is not usually effectively utilized is provided in the ground, by effectively utilizing this.

In the external air taking mechanism for a building, an external air taking-in duct 5b continuing to a suction side of an air conditioner 3 is branched at one end side of the rain water permeation pipe 7 of the rain water permeation trench 6 and an external air taking-in duct 5c in which an external air taking-in port 4b is disposed on the ground is branched at the other end side of the rain water permeation pipe. An external air taking-in duct 5a continuing to an external air taking-in port 4a of the building is branched from the external air taking-in duct 5b at a suction side of the air conditioner. Dampers 10a, 10b for switching a route are provided on the respective external air taking-in ducts.