To provide a floor blow-off port structure with air flow adjustment function that cannot be affected greatly at the time of maintenance or even if external vibration is applied where the amount of air blow-off at a blow-off port can be adjusted easily.

A floor blow-off port 2 is composed of a blow-off port 4 of a lid body 28 that is engaged into an installation hole 26 being recessed on a floor surface 3, a porous cylinder 20 that is drooped into an air circulation space 6 below a floor from the lower part of the blow-off port 4, and a basket 22 that is suspended and supported at the lower edge of the lid body inside the porous cylinder 20 via an exterior thread member 24. The exterior thread member 24 is screwed into an interior thread hole 34 of the basket 22. In the basket 22, a projecting part 22b formed at the outer periphery is engaged into a groove-shaped guide rail 36 on the inner-periphery surface of the porous cylinder for regulating rotation, so that it slides along upper and lower directions due to the rotation of the exterior thread member 24, thus adjusting the increase and decrease in the amount of air blow-off.