We have observed the magnetic domain evolution under an external magnetic field in an epitaxial MnAs film on GaAs(001) by magnetic force microscopy. Owing to the strain involved, the ferromagnetic alpha-MnAs and the paramagnetic beta-MnAs phases coexist as self-aligned stripes at room temperature. It was found that a complex magnetic domain structure appeared in the ferromagnetic alpha-phase region at the demagnetized state (H=0 Oe). As the magnetic field increased, the magnetic domain structure was gradually changed, and reached a completely saturated state at H=600 Oe. Especially, at H=300 Oe, the observed magnetic domain wall distribution is well matched with the topographical shape.