A nonlinear distortion-based ultrasonic diagnostic imaging system displays a raised-resolution video of tissue inside a body at an increased frame rate. Using a two-pulse technique, a transducer driver supplies narrower-width and wider-width driving pulses to a transducer, which transmits weaker and stronger ultrasonic wave pulses alternately while putting the same intervals between adjacent ultrasonic wave pulses to obtain a weaker echo and a stronger echo. An equalizer equalizes each weaker echo to the stronger echo into an equalized weaker echo. An interpolator calculates an interpolation value between the equalized weaker echo and an equalized previous weaker echo obtained from a previous weaker echo. For each weaker ultrasonic wave pulse, a detector finds a difference between the interpolation value and a stronger echo obtained between the weaker echo and the previous weaker echo. The equalization and interpolation enables high-speed scanning, which has not been achieved with two-pulse technique. Thus, a raised-resolution video signal of the tissue is formed at an increased frame rate on the basis of the difference signal and a scan control signal which is also used in the transducer.