1.  Field of the Invention
The present invention relates to a slot machine that increases the number of displayed symbols and a control method thereof. 
2.  Discussion of the Background
There have been conventionally known slot machines that scroll-display and then stop-display a plurality of kinds of symbols and provide a predetermined amount of game media (e.g. a predetermined number of coins, a predetermined amount of money) based upon a combination of the stop-displayed symbols. 
Among such slot machines, there exist slot machines having a symbol array that includes a wild symbol substitutable for another symbol.  Slot machines having the wild symbols are disclosed for example in US 2006/0084498-A1, US 2006/0264254-A1, and US 2008/0070673-A1.  In the slot machine having the wild symbol, when the wild symbol is displayed on a pay line, the wild symbol is substituted for another symbol to form a combination establishing a prize, thus allowing a player to have great expectations. 
The inventors of the present invention has therefore come to consider that adding a new function related to the wild symbol to the slot machine can provide a more attractive game. 
The present invention was made in view of the aforementioned issue and an object thereof is to provide a slot machine capable of providing a more attractive game, and a control method thereof. 
The contents of US 2006/0084498-A1, US 2006/0264254-A1 and US 2008/0070673-A1 are incorporated herein by reference in their entirety.