To provide a distance measuring method by an ultrasonic aerial sensor capable of measuring always highly accurately the distance to a measuring object by an ultrasonic sensor installed in the air.

In this distance measuring method by an ultrasonic aerial sensor, a time until an ultrasonic wave transmitted from the ultrasonic sensor 4 installed in the air is reflected by a free surface (measuring object) of drainage 2 and returned is measured, and the distance L from the ultrasonic sensor 4 to the free surface of the drainage 2 is measured based on the time. In the method a plurality of temperature sensors 5, 6, 7 are installed between the ultrasonic sensor 4 and the free surface of the drainage 2, and sound velocity correction is performed based on the temperature detected by each temperature sensor 5, 6, 7, and the distance L from the ultrasonic sensor 4 to the free surface of the drainage 2 is determined based on the corrected sound velocity.