1. A slot machine comprising: a display having a screen capable of displaying a symbol arrangement image in which a plurality of symbols are arranged and a plurality of valid area images each having a shape capable of including one or more of said symbols; and a controller in communication with the display, said controller determining the number of said valid area images in accordance with the number of BETs, displaying to said screen said symbol arrangement image and the determined number of said valid area images so as to superpose said valid area images on said symbol arrangement image, in such a manner that said valid area images move and then stop, and providing an award in accordance with a symbol included in each of said stopped valid area images so that an award in accordance with a symbol included in an overlap area where the valid area images overlap with each other is higher than an award provided in a case of the symbol included in said valid area image other than said overlap area.