MAXI is an all sky X-ray monitor to be mounted on the Japanese Experimental Module in the International Space Station (ISS). It scans almost all over the sky every 96 minutes, in the course of the orbital motion of the ISS. MAXI is designed to have a sensitivity, significantly higher than the previous X-ray monitors, and then, to detect X-ray sources as faint as 1 mCrab in a week observation. Therefore, MAXI is expected to create a novel catalogue of not only the stable X-ray sources but also the highly variable ones in the sky, especially active galactic nuclei for the first time. If MAXI detects X-ray phenomena, alerts will be quickly made through the Internet.