The 2003 Tokachi-oki earthquake caused the severe damage to oil storage tanks by liquid sloshing. Especially at Tomakomai, two tank fires broke out and six floating roofs sank. Seismograms showed that long-period ground motions were predominant and duration became longer when the seismic waves propagated into the Yufutsu Plain, where Tomakomai is located. Sloshing wave heights (Wh) for all tanks were calculated by the two-dimensional response analysis. It was found that the estimated Wh exceeded 3m at periods 5 and 7.5 sec. and exceeded 2m from 3.5 to 9 sec of sloshing period and that severely damaged tanks had the highest Wh at each period in general.