  We develop a formalism of electric dipole spin resonance (EDSR) based on
slanting magnetic field, where we especially investigate the microwave
amplitude dependence. With increasing microwave amplitude, the Rabi frequency
increases linearly for a spin confined in a harmonic potential. How- ever, when
the spin is confined in the double-well potential, the Rabi frequency shows
sub-linear dependence with increasing the microwave amplitude.
