A magnetic disk apparatus using at least one floating magnetic head which is floated above the surface of a rotating magnetic disk by utilizing dynamic pressure of air caused by the rotation of the magnetic disk, the magnetic disk apparatus comprising, a magnetic head assembly including a magnetic head and a supporting device for supporting the magnetic head to float above the surface of the rotating magnetic disk, the supporting device being provided with a space changing device for changing a space between the magnetic disk surface and the magnetic head, a driver coupled with the space changing device for driving the space changing device, and a controller coupled with the driver for controlling the driving of the space changing device by the driver to change the space between the magnetic head and the magnetic disk surface.