We investigated the migration mode of deep non-volcanic tremor activity beneath Kii Peninsula, southwest Japan. Major tremor episodes are characterized by long-term migration with a velocity of about 10 km/day, propagating along the strike of the subducting plate. Similar tremor migration in Cascadia is accompanied by reverse propagation at speeds on the order of 100 km/day and much faster slip-parallel migration at speeds on the order of 1000 km/day. We systematically searched for migrating tremor with clear linearity in space and time. As a result, we found tremor migrations at speeds ranging from 1 to 60 km/hr depending on the along-dip position in the tremor zone. The observed decrease in migration speed with increasing measurement time scale suggests that migration is controlled by a diffusion process. The along-strike migration at lower speeds, including both forward and backward directions relative to the long-term migration episode, is concentrated at the updip side of the tremor zone, whereas the faster slip-parallel migration is distributed over the entire zone. The long-term migration seems to consist of and be excited by the propagation of along-strike creep at the updip part. The concentration of along-strike migrating tremor sequences at the updip side may reflect the existence of abundant fluid that accumulates at the corner of the mantle wedge. The faster slip-parallel migrations represent projections of along-strike fluctuations in slip pulse propagation controlled by striations along the plate interface.