To provide an operation start device of a game machine which raises amusement during stand-by while making a prize-winning possible timing longer.

When a game ball enters a holding hole 131 of a mobile piece 30 which is supported so as to be changeable, the mobile piece 30 is changed and a game ball is carried to the prize-winning port 21 of a bonus winning main body 20. The prize-winning port 21 is clogged by a stopper board 61, the stopper board 61 is driven by a solenoid 62 in the case of a prescribed timing so as to open the prize-winning port 21 and, then, a prize is won. A display means such as an LED display unit 41, etc., displays an opening timing during a period till prize-winning so that a game player obtains raised amusement by the change of the mobile piece 30 when the game ball enters the holding hole 31 and by the display of the LED display unit 41. Moreover, the holding hole 31 itself is opened till one game ball enters and, then, the prize-winning timing is made to be longer. Thus, amusement is not reduced by half in the game.