The variability of the sea surface height (SSH) around Japan during 1960--2002 was investigated using an ocean general circulation model. The first EOF mode of the simulated SSH change has bidecadal variability and exhibits simultaneous variations around Japan that are in good agreement with the observed sea level changes along the Japanese coast. The variability is caused primarily by the meridional shift of the boundary between the subtropical and subpolar gyres due to shifting of the westerlies over the central North Pacific. The second mode of SSH change indicates a north-south dipole structure around Japan, that results from a change in the strength of the subtropical gyre due to a change in the magnitude of the westerlies. The rising (descending) trend of the sea level observed in the western (eastern) part of Japan in the past 40 years is determined by the increasing trend of the westerlies.