An electric motor device comprising: a housing; a stator fixed to the housing; and a rotor supported by the housing and configured to rotate with respect to the stator, wherein the rotor is a magnetic field mechanism having a permanent magnet, the stator is an excitation mechanism having coils that form a magnetic circuit corresponding to three-phase alternating current, the rotor is configured to be slidable in an axial direction of a rotation axis, the rotor and the stator are arranged such that orientations of magnetic poles are in parallel with the rotational axis, the electric motor device further comprises a rotor slider configured to slide the rotor in the axial direction and a reverse input holder configured to prevent rotation of the rotor in at least one direction, the rotor slider is configured to slide the rotor in the axial direction by an electromagnetic force in the axial direction that acts on the stator and the rotor, the rotor slider is configured to generate the electromagnetic force by a current component generating an excitation magnetic flux in a direction corresponding to an orientation of magnetic poles of the magnetic field mechanism, the reverse input holder includes an engaging portion provided on the rotor and an engaged portion provided on the housing and configured to be engaged by the engaging portion in a state that the rotor is caused to slide in one axial direction by the rotor slider, when the engaging portion and the engaged portion are in engagement, the engagement between the engaging portion and the engaged portion is maintained against a reverse input load that acts on the rotor as an external force, and thereby retains a rotation angle of the rotor, and an axially preloading part for applying preload on the rotor against the housing so as to hold the axial position of the rotor, wherein the rotor slider is configured to slide the rotor in the axial direction against the preload of the axially preloading part to bring the engaging portion in engagement with the engaged portion. a housing; a stator fixed to the housing; and a rotor supported by the housing and configured to rotate with respect to the stator, wherein the rotor is a magnetic field mechanism having a permanent magnet, the stator is an excitation mechanism having coils that form a magnetic circuit corresponding to three-phase alternating current, the rotor is configured to be slidable in an axial direction of a rotation axis, the rotor and the stator are arranged such that orientations of magnetic poles are in parallel with the rotational axis, the electric motor device further comprises a rotor slider configured to slide the rotor in the axial direction and a reverse input holder configured to prevent rotation of the rotor in at least one direction, the rotor slider is configured to slide the rotor in the axial direction by an electromagnetic force in the axial direction that acts on the stator and the rotor, the rotor slider is configured to generate the electromagnetic force by a current component generating an excitation magnetic flux in a direction corresponding to an orientation of magnetic poles of the magnetic field mechanism, the reverse input holder includes an engaging portion provided on the rotor and an engaged portion provided on the housing and configured to be engaged by the engaging portion in a state that the rotor is caused to slide in one axial direction by the rotor slider, when the engaging portion and the engaged portion are in engagement, the engagement between the engaging portion and the engaged portion is maintained against a reverse input load that acts on the rotor as an external force, and thereby retains a rotation angle of the rotor, and an axially preloading part for applying preload on the rotor against the housing so as to hold the axial position of the rotor, wherein the rotor slider is configured to slide the rotor in the axial direction against the preload of the axially preloading part to bring the engaging portion in engagement with the engaged portion.