GHz ultrasonic velocity measurement has been developed up to ～15 GPa in a diamond anvil cell (DAC). In order to perform the GHz ultrasonic measurement, both P and S wave buffer rods were manufactured through sputtering ZnO piezoelectric film on single crystal rods, such as sapphire, rutile (TiO2), and YAG. Several technical breakthroughs have been achieved before observing stable signals from the specimen squeezed in DAC: (1) Wiping by aqueous ammonia on the contact surfaces enhances transmissibility of GHz waves from the buffer rod to the diamond anvil. (2) Cleaning with lathery detergent and water jet effectively removes grime on the rear surface of the diamond. (3) A steel ball supporter allows effortless adjustment of contact between the buffer rod and the diamond. As a result, we succeeded in demonstrating the GHz-DAC ultrasonic velocity measurement on KCl and natural olivine samples.