A least squares inversion technique was applied to Rayleigh waves with periods of 10--25 s to determine the distribution of moment release within the source region of the 1968 Tokachi-Oki earthquake. The inversion procedure attempts to produce synthetic seismograms which match the data by solving for the amount of moment generated from each point of a grid spaced at 40 km. The data were recorded on low-gain mechanical seismometers that were located 70--200 km from the source region. Assuming simple unilateral and bilateral rupture models, the inversion was run for different values of constant rupture velocity, resulting in a best fit to the data at 3.0--3.1 km/s for both models. The calculated surface wave synthetics depended strongly on the crustal velocity model. By using a velocity structure obtained from a refraction survey, the waveforms of the synthetics became significantly different, depending on whether we used discontinuities or linear gradients to connect the depth points for which the survey gives a refractor P velocity. Neither a structure of only discontinuities nor a structure of only gradients was found to produce satisfactory synthetics. Therefore an intermediate structure with a linear gradient near the surface and discontinuities below was used. The results of the inversion show that the data can be modeled with a source consisting of two strong subevents which dominate the radiation of these surface waves. The relation location of the two subevents is consistent with the locations of the two subevents of high stress drop that were seen on strong motion records.