Motion of beam electrons in a spiral undulator with an axial guide magnetic field is investigated analitically. It is shown that the projections of electrons trajectories in the cross-section plane are in general case ellipses. Such motion can be represented as the superposition of two, clockwise and counter-clockwise, circular girations. Because of this the cyclotron resonance arises for both directions of the guide magnetic field orientations, conventional and inverse. The "anticyclotron" resonance is conditioned by the difference between the amplitudes of radial and azimuthal components of the undulator magnetic field for off-axis electrons.