We are developing a superconducting magnet system to grow high-quality protein crystals. The gravity-controlled environment, based on magnetic forces, can suppress thermal convection and may give rise to a variety of additional effects on the protein crystal growth. To design suitable magnetic force conditions for protein crystal growth in protein solutions, we are studying a gravity-controlled environment by magnetic forces in the crystal growth process by computer simulations. In this study, we derived a modified Navier-Stokes equation with gravity and static magnetic force and numerically solved the equation. The obtained results show that the temperature dependence of the magnetization modifies the levitation condition and the magnetic force gives rise to an unexpected change of fluid motion.