High transfer sound pressure and high reception sensitivity are realized, and reliability is improved in terms of long term operation, in a capacitive detector-type ultrasonic transducer (CMUT). The ultrasonic transducer, which has a lower electrode (201), a hollow portion (202) that is formed on the lower electrode and surrounded by insulating films (209,208), an upper electrode (205) that is formed on the hollow portion, and a plurality of insulating film projections (204) that are formed in the hollow portion (202), comprises a plurality of rigid members (203) that are formed on the hollow portion, either the lower electrode (201) and/or the upper electrode (205) is disposed in a position that does not overlap with the insulating film projections (204) when viewed from the upper surface by carving out the portion that overlaps with the insulating film projections (204), and the respective rigid members (203) are disposed such that a region is present that overlaps with the insulating film projections (204) when viewed from the upper surface.