A magnetic field was applied with permanent magnets to the natural convection of oxygen-containing air in a cubic container consisting of a pair of vertical walls maintained at different temperatures. Two-dimensional numerical simulations were performed on this configuration to investigate the change in heat transfer due to the arrangement and shape of the prismatic magnets. The magnets were placed in all four directions of the container: top, bottom, left, and right. When a magnet is placed on the bottom or left side (hot wall side) of the container, the heat transfer is either enhanced or suppressed, though the effect is subtle. On the other hand, when a magnet is placed on the top or right side (cold wall side) of the container, the heat transfer is significantly suppressed, relying on the position of the magnet. A magnet placed near the cooling wall excludes hot oxygen, adsorbs cold oxygen, and forms vortices on the surface of the cooling wall, thereby blocking the approach of hot air. Unlike the heat transfer suppression by an adiabatic wall, this heat transfer suppression mechanism acts directly on the working fluid, keeping the surface of the cold wall at a low temperature.