An echo signal reflected from a microbubble contrast agent is discriminated from an echo signal generated upon reflection of a nonlinearly propagated transmission pulse from the body tissues without degradation of the axial resolution, by performing transmission/reception twice or less which would hardly decrease the imaging speed. By detecting a difference in phase of the second harmonic component based on the fundamental component included in the echo signal, an echo signal generated upon nonlinear reflection from a microbubble contrast agent is discriminated from an echo signal generated upon linear reflection of a nonlinearly propagated transmission pulse from the body tissues. The phase of the second harmonic component is detected through phase sensitive detection in which the square of the fundamental component is used as a reference wave. Concurrently, a pulse inversion method is used to extract the second harmonic component included in the original echo signal, whereby degradation of the axial resolution is prevented.