Onchidium pacemaker (OP) neurons show oscillatory behaviors such as beatings and periodic or chaotic discharges. Hayashi et al. proposed a set of modified Hodgkin-Huxley equations as a model of the OP neurons and succeeded to reproduce the discharge patterns. The model consists of 8 variables including those of fast and slow potasium and sodium currents. It is difficult, however, to gain mathematical insight of the model because of high dimensionality. Recently, Kepler et al. proposed a systematic method to reduce a number of variables in H-H equations without loss of essential features of the dynamics. In the present work, we apply the method by Kepler et al. to reduce the model of OP neurons by Hayashi et al. and show that the reduced one mimics well the oscallatory behaviors of the OP neurons.