A unique data set of simultaneous observations of the ocean current and absolute sea surface slope in the Tsugaru Strait, Japan, has been analyzed in order to examine the effect of Coriolis force on an ocean current. First, the mean surface slope across the strait (an average over the period 1972--1980) was compared dynamically with the mean ocean current along the strait (an average over the period 1953--1978). It was found that both are in good geostrophic balance. Next, time variation of the ocean current was compared with that of sea surface inclination. In the subtidal frequency range, the variation in current velocity was found to be followed by the variation in sea surface inclination such that both are generally in geostrophic balance. Although some imbalances are seen, most of them can be explained by the sea level changes caused by the storm surge or wave setup. In the tidal frequency range, the dynamical relation between the current velocity and inclination shows good agreement with the theory. The downstream component of the balance is maintained mostly by the local acceleration and pressure gradient terms, and the cross-stream component is maintained by the Coriolis force and pressure gradient terms. ¿ American Geophysical Union 1989