Hayashi et al.presented a Hodgkin-Huxley type neuron model with eight variables as the model of mollusk Onchidium pacemaker neurons which showed several oscillatory phenomena such as beating and periodic or chaotic bursting discharges.In order to study the dynamical behavior of the pacemaker neuron model,we derive a three- variables reduced model from the original neuron model using the reduction method proposed by Kepler et al..Then,using the nullcline in the three-dimensional phase space,structures of the periodic orbit corresponding to beating and bursting discharge are studied.