The present invention relates to a power transmission device for a water pump, including a pulley, a magnetic flux generator, a disk assembly, and a hub assembly, wherein the disk assembly has a disk, a friction ring for providing a frictional contact with the hub assembly, and a resilient ring adapted to pressurize the friction ring toward the hub assembly, so that when the attracting magnetic flux of the magnetic flux generator is not applied, the friction ring makes a frictional contact with the hub assembly through the resilient pressurization of the resilient ring, whereas, when it is applied, the disk moves toward the pulley to allow the frictional contact between the friction ring and the hub assembly to be released.