The MnAs spin system has been modelled as a three-dimensional Ising system under the assumption that the exchange interactions are independent of the external conditions (i, e. a rigid spin system). The magnetic behaviour of this system in magnetic fields of 20, 40, 60 and 90kOe has been studied. The order parameter m = M/M\ (where M is magnetisation) has been obtained by Monte Carlo simulation of an 8 x 8 x 6 spin system with hexagonal symmetry under periodic boundary conditions. The relaxation behaviour of m near Tc shows that the model system undergoes a first-order phase transition from a ferromagnetic into a paramagnetic state. The resulting magnetic phase diagram is in good agreement with the experimental one. The independent behaviour of the magnetic subsystem is shown to play an essential role in the transition mechanism.