We have devised a new type of simple acoustic focusing device with an interdigital transducer in which all radiation from each section of the transducer constructively interferes at a focal point in water. The position of the focal point of the device is variable by changing the carrier frequency of an electrical signal applied to the transducer. The device is applicable to the acoustic imaging in both transmitting and reflecting modes. This paper reports basic design considerations for the transducer and discusses operational performance.