PURPOSE: To suppress the formation of a false image, by improving a transmission wave form to suppress grating side lobe.

CONSTITUTION: A sound wave comprising two cycles of a positive phase with amplitude of A and a wavelength of λ and a reverse phase is transmitted from each ultrasonic convertor element. The sound wave from each element in the vicinity of grating side lobe is transmitted so as to be shifted at every one wavelength and a synthetic transmitting surface comes to a wave wherein two phases are mutually separated and the total length is brought to (N+1)λ. A receiving wave form in a grating side lobe direction comes to one shown by (g) in the drawing if reflective capacity is supposed as 1. Because both of the synthetic transmitting wave (f) and the receiving wave (g) have a wave height value of A, grating side lobe is improved to A/N2A=1N2 (N is the number of elements).