The present invention provides a barbecue grill assembly with a grill body having a firebox and a cover and a burner element. A gas introduction hole for introducing gas containing water vapor is formed in another predetermined location, other than in the raised portion, in the inside surface of the wall portion. The first polarizer has an absorption axis inclining by about 45 degrees relative to the lagging axis of the first retardation plate, and inclining by about 90 degrees relative to an absorption axis of the second polarizer.