Materials having different compositions are arranged to prepare a molded form in which the sound velocity characteristic is varied from high frequency to low frequency from the center toward the periphery. An ultrasonic wave-radiating face of the molded form and the back face of the ultrasonic wave-radiating face are arranged in parallel to each other to prepare a transducer in which the center region can transmit/receive high-frequency ultrasonic waves and the periphery region can transmit/receive low-frequency ultrasonic waves.