A valve case mounting structure includes a mount base and a valve case. The mount base includes a valve case support portion and a retainer piece having a first engagement opening and a second engagement opening. The valve case includes an engaging projection portion, and a holder portion. When the engaging projection portion is positioned at a front end side of the first engagement opening, the retainer piece does not reach the holder portion. When the engaging projection portion is moved from the front end side of the first engagement opening toward the second engagement opening, a front end portion of the retainer piece is inserted into the holding space before becoming unable to be inserted into the holder portion.