An adapter for use in puncture (a puncture adapter) for performing puncture while an ultrasonic image is being observed. The puncture adapter is composed of a holder for holding a probe which transmits and receives an ultrasonic wave, and a base unit for movably supporting the holder. A guide mechanism provided in the base unit guides a puncture needle. Moving the probe held in the holder moves a beam scan plane, and the intersection between the beam scan plane and a puncture route can be moved along the puncture route. The base unit functions as a seat which is brought into contact with a surface of a living body. A plurality of projecting legs may be provided on the base unit. The plurality of projecting legs are provided along the periphery of the movement region in which the wave transmitting and receiving surface of the probe moves.