To form a synthesized drive signal having an accurate delay time, based on a drive signal which has discrete delay time supplied to an ultrasonic probe.

When an ultrasonic pulse is transmitted to the inside of a subject's body by actuating Mx pieces of vibration elements which are two-dimensionally arranged in the ultrasonic probe 3, a signal choice part 32 of the ultrasonic probe 3 chooses, to each of the vibration elements, two contiguity drive signals having a delay time which is the nearest to the accurate delay time (ideal delay time) required for driving the vibration elements among the drive signals of an Mo (MoMx) channel supplied from the transceiving part 2 of an apparatus body 1 in a state that the delay time is quantized by Δτ. Then, a burden attachment part 33 performs weighting processing using a variable capacity device to each of the selected contiguity drive signals and a synthetic-distribution part 34 forms the synthesized drive signal, which has the ideal delay time, by addition composing the contiguity drive signal which is weighting processed.