Microbubbles with diameters smaller than 100 mu m are currently attracting considerable attention because of their properties such as a large surface area per unit volume, low rise velocity, and self-pressurization due to surface tension. Because of these properties, they find wide applications in agriculture, aquaculture, and medicine. Makuta et al. pioneered the development of a microbubble generator using a hollow cylindrical ultrasonic horn. In this method, the gas supplied through the path in the horn creates a gas-liquid interface in water, and microbubbles with diameters smaller than 100 mu m are easily generated by irradiation of the ultrasonic oscillation to the gas-liquid interface. However, a higher yield of microbubbles is required to enable their practical application. Therefore, we changed the orifice shape by replacing the detachable tip at the end of the hollow cylindrical ultrasonic horn and evaluated the yield of microbubbles from the dissolved oxygen concentration in water for different shapes of the orifice. We also investigated the microbubble generation ability of the generator under changing flow rates of the gas supply to the horn. We found that the microbubble generation ability improved with an increase in the number of orifices at the horn end.