Numerical studies on particle velocity and on sound pressure by circular flat transducers have been carried out, and the leaps of the phase delay of the sound pressure on the axis of the transducer and those of the specific acoustic impedance on it have been investigated [T. Imamura, J. Acoust. Soc. Am. 98, 2876(A) (1995)]. In the neighborhood of the central axis of the circular flat transducer, the leaps of the phase delay of the sound pressure and those of the specific acoustic impedance disappear at points off the central axis. In the very vicinity of the axis, the phase delay of the specific acoustic impedance increases sharply from minus to plus, where the amplitude is the minimum. According to the distance from the central axis of the circular flat transducer, the feature of the phase delay gradually fades out. In conclusion, the ultrasonic field on the axis of a circular flat transducer is the extremity of the ultrasonic field in the neighborhood of it. From these results, particle velocity and specific acoustic impedance of the ultrasonic field are revealed to be very important for analysis.