The slip distribution of the 2003 Tokachi-oki earthquake is estimated from the 11 tsunami waveforms recorded at 9 tide gauges and 2 ocean bottom tsunami-meters (pressure gauges) off Kamaishi, Tohoku, using the waveform inversion technique. The plate interface of the source area is divided into 48 subfaults. Smoothness constrains are applied to stabilize the inversion. Large slip amounts, more than 3m, are estimated on the subfaults located east of Hiroo. The total seismic moment is 1.1×10²¹ Nm (M_w8.0). The slip distribution estimated from the tsunami waveform inversion is similar to the slip distribution estimated by Yamanaka and Kikuchi (2003) using the teleseismic body waves.