Evidence of a slow slip temporally ahead of the M7 earthquake doublet that occurred in November/December 2004 offshore of eastern Hokkaido, Japan, was registered in continuous GPS measurements. The GPS data suggest that a slow slip around a forthcoming rupture zone on the plate boundary triggered these earthquakes. This slow slip episode was itself triggered by the M8 Tokachi earthquake of September 2003. This is a demonstration of the geodetic visualization of the mechanism of a triggering of a subsequent earthquake caused by a preceding large earthquake. The geodetic data will improve the reliability of the warning of an occurrence of a subsequent earthquake after a large one that is currently dependent on only co-seismic static stress increase.