A foam pump dispenser that has a prevention function against any reverse flow is disclosed, which comprises an air guide which is engaged at a lower side of the air and liquid mixing chamber of the piston cover member; a liquid leakage prevention spring which is located between the piston cover member and the piston member; and a liquid leakage prevention valve which is covered on the upper side of the liquid compression piston. The foam pump dispenser with the above construction is able to prevent a leakage of liquid, and is able to obtain a reliable fixed supply of air in a pump dispenser designed to properly mix liquid such as foaming cleanser, foam hand soap, shave foam, and hair mousse, etc. with air and to pump out in a foam shape.