To provide an ultrasonic flow velocity measuring method and an ultrasonic flow velocity measuring instrument capable of measuring the flow velocity by switching a sensor corresponding to turbidity.

A first ultrasonic sensor S1 is arranged on one side shore of a river 1, and a second ultrasonic sensor S2 is arranged on the opposite shore on the position separately as long as a prescribed distance along the flow from the first ultrasonic sensor S1, and a third ultrasonic sensor S3 is arranged at a shorter distance than the prescribed distance along the flow on the same shore side. This method is characterized by measuring the flow velocity between the first ultrasonic sensor S1 and the second ultrasonic sensor S2 when the water turbidity is small, and measuring the flow velocity between the first ultrasonic sensor S1 and the third ultrasonic sensor S3 when the water turbidity is large.