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In this paper, the method of 3D transient analysis of the ultrasonic propagation in flow field is developed using the finite difference time domain method. In this method, in order to take account of flow field, the semi-Lagrange coordinate system, in which physical quantities are defined by the Euler coordinate system and the time derivative term is discretized by the Lagrange coordinate system, is applied. The developed method is applied to a simple model of flow meter for air, in which the flow fields are laminar and turbulent flows. To verify the developed method, the measurement is also carried out. The waveform of acoustic pressure on the surface of receiver obtained from the calculation is compared with the measured voltage waveform. The propagation times obtained from the calculation are in good agreement with those obtained from the experiment in various flow rates. However, the tendency of amplitude of waveform by flow rate in the calculation is different from that in the experiment at present.