A water dispenser is provided which includes a cold water tank, a raw water pumping pipe through which a raw water container communicates with the cold water tank, a pump provided at an intermediate portion of the raw water pumping pipe, a buffer tank provided beside the cold water tank, a buffer tank supply pipe through which the air layer in the buffer tank communicates with the cold water tank, a hot water tank provided below the buffer tank, a hot water tank supply pipe through which the buffer tank communicates with the hot water tank, first and second three way valves provided in the raw water pumping pipe, a first sterilization pipe through which the first three-way valve communicates with the buffer tank, and a second sterilization pipe through which the second three-way valve communicates with the hot water tank.