To make a player enjoyable the amusement of the probability change by setting a condition generable irrelevant to the special play situation as the condition of the probability change.

When a ball enters either of start winning holes 4a-4c, a dot matrix LED of a pattern display device 25 starts scroll fluctuation at such speed as unrecognized by a player and after a prescribed time has elapsed, the fluctuation display is stopped and displays three patterns. When the combination of the stopped and displayed patterns is the combination of big win patterns, it becomes a big win play state, or one of the special play states. In the big win state, opening/closing members 27a, 27b are opened for a prescribed time. Then, when it becomes the big win during the number of winning balls entered in the start winning holes 4a-4c, for example, reaching 50 from the charge of the power supply in opening a parlor, the probability of emerging the big win patterns in the next fluctuation and after is improved irrelevant to the combination of the big win patterns.