A back flow preventer for preventing contamination of a water supply. The preventer includes check valves which are enclosed in a casing comprised of a body and a single removable cover. The check valves are vertically disposed and spaced laterally from each other. The valves may be removed for servicing when the cover is removed without disturbing other parts of the device. Between the check valves is a relief valve which opens to drain the passage between the check valves when there is a drop in the supply line pressure sufficient to cause actual or potential reverse flow back toward the supply line. The check valves open by movement in opposite directions. Each check valve unit includes a tubular strainer through which all water must pass. The check valve units, while identically dimensioned as to the valve structure, include other parts of unequal dimensions which preclude accidental reversal of the units when being initially assembled or replaced after servicing.