To permit efficient air conditioning without giving workers in a room any cold draft feeling.

An inducing and mixing device 18 is provided in an underfloor space 26 whereby air-conditioning air A to be supplied to the underfloor space 26 is supplied into the underfloor space 26 under the condition of being mixed with indoor air B. Accordingly, when cooling air conditioning is effected in a room 12, for example, air-conditioning air is supplied into the underfloor space 26 under a condition that cold air-conditioning air A is mixed with warm indoor air B and, thereafter, mixed air C is sent into the room 12 whereby any cold draft will not be generated in the room 12. The temperature of the air-conditioning air A can be lowered than a conventional case whereby a direct expansion type cooler, capable of effecting efficient air conditioning, can be used as the cooler 32 of an air conditioner 16 for producing the air- conditioning air A.