Snow hydrological studies were carried out in two experimental watersheds in Hokkaido to examine the mechanism of spring snowmelt runoff during a rain-onsnow event. A study of the runoff process within the snowpack showed that not less than 90% of the water that flowed out from the bottom of the snowpack had been stored within the snowpack before the event, and that this rate did not vary much based on whether it was a snowmelt-only event or a rain-on-snow event. A study of the peak streamflow under a rain-on-snow event showed that the peak streamflow was caused by the large initial streamflow just before the hydrograph rise and by the large inflow intensity comprised of the rainfall intensity and the snowmelt intensity. Furthermore, an artificial heavy rainfall on the snow surface was experimentally simulated, and although 200 L of water was sprinkled on 1 m² of snow surface for 6 hours, no outflow appeared from the bottom of the snowpack. This result indicated that a horizontal flow component became remarkable beyond expectation when a lot of water was supplied over the snow surface. To clarify the detailed processes of spring snowmelt runoff under a rain-on-snow event, it is crucial to know how much water is supplied to the ground surface under the snowpack. To do so, we need to accumulate more observation results regarding the amount of snowpack bottom outflows using a snow lysimeter.