Purpose: Acoustic lenses have generally been used in probes of ultrasonic diagnostic equipment and are primarily composed of silicone rubber. Manufacturers are now conducting material research on these lenses considering the use of in the air. This study considers the utilization of a phononic crystal structure as a new device for acoustic lenses used as probe, in order to improve the performance of ultrasonic probes for ultrasonic diagnostic equipment. Subjects and Methods: To determine the basic properties of a conventional planar acoustic lens that uses water as a filler and an acoustic lens that uses silicone rubber as a filler, we obtained the acoustic focusing field of the lenses using the two-dimensional elastic finite-difference time-domain method as the numerical analysis method. Furthermore, we obtained the focal distance, beam width, and frequency characteristics in order to clarify various properties of an acoustic lens. Results and Discussion: Analysis of the acoustic focusing field of the planar acoustic lens with silicone rubber clearly showed that the planar acoustic lens focused ultrasound when the radiation frequency of sound was varied from 2.5 MHz to 3.2 MHz. The frequency range of convergence shifted to the low frequency compared with that of the planar acoustic lens with water. The frequency shift results from the low limiting basic frequency between crystalline lattices that is caused by the slow sound velocity of silicone rubber compared with the sound velocity of water. Conclusion: We confirmed the expansion of frequency bands of the planar acoustic lens with a phononic crystal structure constructed with silicone and stainless steel rods considering the use of in the air. In future studies, we plan to analyze the precise sound distribution in three-dimensions using the three-dimensional elastic finite-difference time-domain method.