Results of in situ observations on the growth of the faceted primary naphthalene plate during unidirectional solidification of the camphor-naphthalene eutectic system are described. The growth of primary naphthalene plate is characterized by a large fluctuation in growth velocity, while the primary camphor dendrite grows consistently with nearly constant velocity. Discussions were made on the discontinuous growth behavior on the basis of the relation between the undercooling and the frequency of generating atomic steps at the solid/liquid interface.