We have studied the orientational distributions and rheological properties of dilute colloidal dispersions consisting of ferromagnetic spherocylinder particles, for the case of a steady simple shear flow in a magnetic field parallel to the shear flow direction. The equation governing the orientational distribution function was first derived, and it was then solved approximately by Galerkin's method. With these numerical solutions, we obtained the results of the orientational distribution and viscosity. In the case of a weak magnetic field, the particles tend to orient in two mutually opposite directions. As the magnetic field increases, the rotation of the particles is restricted and the viscosity increases significantly. As the influence of the magnetic field becomes dominant, an overshoot in the viscosity curve appears. This is because there is a maximum deviation of the averaged particle direction from the magnetic field direction. When the strength of the magnetic field increases overwhelmingly, the particle inclines close to the magnetic field direction and the viscosity converges to a constant value. The particle of a larger aspect ratio leads to the larger increment in the viscosity, since such elongated particles induce larger resistance in a flow field.