In a discharge lamp apparatus for lighting a discharge lamp, a ballast is disposed outside of a lamp housing of a discharge lamp and is electrically connected to the discharge lamp through a connector. A ballast housing of the ballast is connected to a bottom wall of the lamp housing. In the ballast housing, a ballast circuit board is arranged along a top wall of the ballast housing that is opposed to the lamp housing. Accordingly, it is less likely that the ballast circuit board will be wetted by condensation or water vapor entered the ballast housing. Additionally, a partition wall can be provided in the ballast housing between the connector and the ballast circuit board to block the condensation or water vapor from flowing toward the ballast circuit board.