Ultrasound is widely applied in the clinical field. It is essential to take a real understanding of dynamics of micro bubbles in these applications. But it has not been clarified yet. In this paper we numerically simulate a single micro bubble and bubble cloud, which consists of micro bubbles, in water. In the case of a single bubble, we conduct FFT analysis of acoustic pressure emitted from the bubble in ultrasound field, and we investigate the amplitude of second-harmonics and sub-harmonics. In the case of bubble cloud, we observe that pressure at the center of cloud drastically changes depending on the frequency of ultrasound.