A capacitive micromachined ultrasonic transducer comprising at least one cell that comprises: a first insulating film and a second insulating film disposed with a gap therebetween; a first electrode and a second electrode disposed on outer surfaces of the first and second insulating films , respectively, with the gap therebetween; the at least one cell having an electrostatic capacitance between the first and second electrodes that varies with a variation of a thickness of the gap caused by displacement of the second insulating film and the second electrode ; and voltage applying means arranged to apply a voltage to between the first electrode and the second electrode in order to generate electric fields in the first insulating film and in the second insulating film , wherein the first insulating film is formed of silicon oxide, and the second insulating film is formed of silicon nitride, the cell further characterized in that : the electric field strength generated in the first insulating film is closer to an electric field strength that causes dielectric breakdown of the first insulating film than the electric field strength generated in the second insulating film to an electric filed strength that causes dielectric breakdown of the second insulating film, and wherein a thickness of the second insulating film is greater than or equal to 0.32 × G 1.24 , where G represents the thickness of the gap . a first insulating film and a second insulating film disposed with a gap therebetween; a first electrode and a second electrode disposed on outer surfaces of the first and second insulating films , respectively, with the gap therebetween; the at least one cell having an electrostatic capacitance between the first and second electrodes that varies with a variation of a thickness of the gap caused by displacement of the second insulating film and the second electrode ; and voltage applying means arranged to apply a voltage to between the first electrode and the second electrode in order to generate electric fields in the first insulating film and in the second insulating film , wherein the first insulating film is formed of silicon oxide, and the second insulating film is formed of silicon nitride, the cell further characterized in that : the electric field strength generated in the first insulating film is closer to an electric field strength that causes dielectric breakdown of the first insulating film than the electric field strength generated in the second insulating film to an electric filed strength that causes dielectric breakdown of the second insulating film, and wherein a thickness of the second insulating film is greater than or equal to 0.32 × G 1.24 , where G represents the thickness of the gap . the electric field strength generated in the first insulating film is closer to an electric field strength that causes dielectric breakdown of the first insulating film than the electric field strength generated in the second insulating film to an electric filed strength that causes dielectric breakdown of the second insulating film, and wherein a thickness of the second insulating film is greater than or equal to 0.32 × G 1.24 , where G represents the thickness of the gap . <EOS>