The oceanic crust is in contact with sea water, and thus water is likely to penetrate downward into the solid earth. Meanwhile, the oceanic crust is a place where water is partitioned from the magma. Water that is incorporated into the oceanic crust as hydrous minerals is mainly 1. water in differentiated magma, 2. water released from subducted oceanic plate and supplied to magma via wedge mantle in the back-arc spreading system, and 3. sea water causing hydrothermal circulation. Here, I give an overview of behavior of the water partitioned from the magma into the lower oceanic crust, which is of particular difficulty to access, and the actual state of water is difficult to grasp. In addition, I present the deep-rooted hydrothermal circulation reaching to the lower oceanic crust.