A valve unit includes a housing which has a partition wall in which an opening portion is formed, a float valve which is disposed within a valve chamber R1 thereof, and a check valve which is disposed within a valve chamber R2. A valve seat is provided on a lower surface side of the partition wall, and plural radially-extending thin ribs are formed therearound. Plural axially-extending check valve guide ribs are provided at predetermined circumferential intervals with in the vent chamber R2, and plural axially-extending float valve guide ribs are provided at predetermined circumferential intervals within the valve chamber R1. The thin ribs and the check valve guide ribs are formed circumferentially alternately so as not to be superposed on each other when viewed from an axial direction of the housing.