To minimize a motor-driven water pump by applying a pressuring magnet capable of sliding or oscillating in a storage space for pressurization and decompression by using the magnetic attraction force and the magnetic repulsive force in accompany with the rotation of a rotor or the switching of the power supply to a coil.

In the case when the cooling water is circulated through a radiator, the power supply to each coil 8 of a motor-driven water pump 1 is controlled by a controller 11, and when a magnetic pole is formed on a projection part 7a a rotor 6 is rotated by the action of the magnetic attraction force between the projecting part 7a and the permanent magnet 4. On this occasion, the intensity of the magnetic attraction force and the magnetic repulsion force is controlled by controlling the intensity of the electric current to be supplied, whereby the desired torque can be obtained. The cooling water taken from an inlet 2a is discharged from a discharge port 2b by rotating an impeller 5 by the rotation of this rotor 6.