It is predicted that the earth's magnetic field can be significantly perturbed locally by the microwave beam transmitted from the conceptualized solar power satellite (SPS) at a frequency of 2.45 GHz with incident power density of 230 W/m2 at the center of the beam. The simultaneous excitation of earth's magnetic field fluctuations and ionospheric density irregularities is caused by the thermal filamentation instability of microwaves with scale lengths greater than a few hundred meters. Earth's magnetic field perturbations with magnitudes (~a few tens of gammas) comparable to those in magnetospheric substorms can be expected. Particle precipitation and airglow enhancement are the possible, concomitant ionospheric effects associated with the microwave-induced geomagnetic field fluctuations. Our present work adds earth's magnetic field perturbation as an additional effect to those such as ionospheric density irregularities, plasma heating, etc., that should be assessed as the possible environmental impacts of the conceptualized solar power satellite program.