An ultrasonic multiphase flowmeter, a flow rate measurement program and a multiphase flow rate measurement method using an ultrasonic wave that can measure the flow rate of a multiphase flow by detecting the position of an interface between phases by an operation processing of at least one of data on reflected ultrasonic wave and data on a flow velocity distribution are provided. The ultrasonic multiphase flowmeter functions to transmit/receive ultrasonic waves, calculate flow velocity distributions, determine interface positions, and calculate flow rates.