In recent years, there has been a growing demand for micro holes. However, electrochemical machining has rarely been employed in drilling these holes because of problems with electrolyte diffusion. In this research, a semi-cylindrical tool was used as a tool electrode to increase the flow space of the electrolyte, and electrolyte diffusion was improved via the application of ultrasonic vibrations. Micro holes with a specified diameter of $76{mu}m$ were drilled on a 304 stainless steel plate of $300{mu}m$ thickness. The proposed technique reduced both the machining time and the machining gap.