To specify a real trusted customer and a player causing a disadvantage to a game parlor from a numeric value calculated based on a past game record of a player.

A management computer 30 collects game data from each game stand 10 and each lending machine 20, and calculates a gross margin ratio of customers for each player (for each member) based on cumulative sales and the amount of money for a cumulative difference in the number of balls. The gross margin ratio of customers is stored and saved in databases 31-4 for gross margin ratio of customers of the management computer 30, and the gross margin ratio of customers is displayed on a display part 35 of the management computer 30. Moreover, an abnormal gross margin ratio of customers is displayed on the display part 35 by comparing a value determined as an abnormal value with the gross margin ratio of customers.