To improve prediction accuracy of snow melt runoff.

This snow melt runoff prediction system 100 receives measured meteorological data including rainfall data before the current state in a target basin and predicted meteorological data after the current state with a data collector 200. In a data arithmetic device 300, a snow melt calculation section 320 calculates the water amount that flows down on a snow cover layer from a snow surface and is supplied to the soil based on the received meteorological data, and an outflow calculation section 330 calculates the predicted outflow amount using an outflow model based on the calculated water amount supplied to the soil. The snow melt calculation section 320 calculates water amount supplied to the soil based on the snow melt amount and rainfall using a snow cover infiltration model where the snow melt water infiltrates into the snow cover layer and rainfall directly arrives at the soil without infiltration into the snow cover layer.