To clarify the formation of a sedimentary basin and its groundwater flow system is one of the most important issues in hydrogeology. The Tokachi Plain groundwater basin has been formed in close relation to the sedimentary basin formation. The aquifers are approximately composed of six layers, i.e., a phreatic aquifer, four confined aquifers of upper Pliocene to Pleistocene and a lower Pliocene hot spring aquifer. Hydrogeological surveys such as temperature, stable isotopic ratios and quality of groundwater, hydraulic analyses and three-dimensional thermal advection-dispersion analysis so far revealed the following items: Groundwater in the Tokachi Plain is of meteoric origin, which is recharged at the piedmont area of the surrounding mountains and flows into the central area of the plain. The stable isotopic ratios δD and δ¹⁸O of the Tokachi Plain agree with the regional groundwater flow system model, indicating lighter isotopic ratios in deeper aquifers. This reflects the altitude isotope effect on surrounding mountains. The eastern geological block of the Tokachi central fault forms an eastern impermeable boundary of the Tokachi artesian basin, except the area on or around the fault with relatively high permeability. The Tokachigawa hot spring water is formed as the convergence of groundwater flow in the western Tokachi Plain. Dissolved organic matter analysis supports these results. In addition, groundwater levels of the Tokachi Plain hot springs show noticeable coseismic responses to the giant earthquakes around Hokkaido. The present review discusses the effectiveness of environmental tracers and changes in groundwater level to clarify a groundwater flow system, the Tokachi Plain, Hokkaido.