Recent development of super-conducting materials at high temperature has got enabled to use a super-conducting magnet of 5-10 Tesla and various new phenomena have been reported. In the present report, convection is considered for air containing oxygen whose magnetic susceptibility is exceptionally large among gases. Air is filled in a cubic enclosure whose one vertical wall is kept at a higher temperature and an opposing wall is cooled at a lower temperature and other four walls are thermally insulated in the presence of a gradient magnetic field produced by a one-turn coil located above the enclosure. A numerical model including the magnetizing force and the gravitational force was developed and numerical computations were carried out for the parameters, Pr=0.71, Ra=10(5)-10(6), gamma=0-10 to represent the strength of magnetic field versus the gravitational potential. For example, at Ra=10(5) the average Nusselt number 4.476 at gamma=0 (with no magnetic field) increased to 5.160 at gamma=10. This system was further studied on the effect of inclination with respect to a horizontal axis at the bottom of the hot wall. With an application of a magnetic field, the conduction state heated from above at an inclination angle of 1.5pi radians was found to change to a convection state. Flow visualization experiments were further carried out for the same size of a cubic box attached at an outlet of a bore of a super-conducting magnet of 10T with 100 mm diameter. The magnetic field induced the convection for the system heated from above and cooled from below. The computed now mode agreed well with the experimental one to support the computed results favorably.