Temporal and spatial variations in the inflow of oceanic water into Tokyo Bay and the accompanying changes in circulation within the bay have been studied by repeated measurement of the longitudinal distributions of hydrographic parameters and oxygen concentration from the bay head to off the bay mouth. Data collection took place in summer 1998 with the particular aim of tracking the movement of a hypoxic water mass. Density changes in the upper ocean induce inflow and outflow of oceanic water in the bay so that when water density increased in the upper ocean, heavy saline water intruded into the lower layer of the bay. This inflow forced a bottom hypoxic water mass to the bay head and, finally, lifted it up to form a subsurface hypoxic water mass, which then moved out as a thin layer just under the pycnocline, reaching beyond the bay mouth. When the intruding heavier deep water retreated and an intrusion of oceanic water at intermediate depth occurred, the hypoxic water mass was displaced from the center to the mouth of the bay. The bottom hypoxic water mass dominates in the northern half of the bay, while the subsurface oxygen minimum appears frequently in the southern half.