An inundation simulation program and device using the same for displaying a map, receiving input from a user indicating a starting point and an ending point in a bank breakage section on the map, and displaying on the map a line connecting the starting point and the ending point. Further, the computer program and device calculates a direction of a flow quantity in the bank breakage section by calculating a normal line to the side of the inside area of the bank at the input band breakage section and making the bank breakage section discrete in the calculation grids of the inundation simulation.