The influence of an external magnetic field on microstructures in a colloidal dispersion composed of rod-like ferromagnetic particles was investigated by means of the cluster-moving Monte Carlo method. The internal microstructures obtained by simulation were analyzed in terms of the orientational distribution functions and the pair correlation functions. The results obtained are summarized as follows. As the magnetic field increases, the particles align in the direction of the magnetic field. In the case of a relatively strong magnetic interaction between particles, chain-like clusters are formed along the magnetic field. However, the aspect ratio of the particles and the magnetic interaction between particles do not affect the orientational distribution of the particles. The chain-like clusters are observed to have two types of structure: a straight linear structure and a step-like structure. The clusters become shorter when the area fraction of the particles decreases, while the number of step-like structures increases when the area fraction of the particles increases. The step-like structure formation can be explained by the potential energy curves depending on the shape of the spherocylinder particles.