C. elegans has been widely studied for understanding the basic mechanisms of nervous system function. In order to examine neural activity in C. elegans, it is necessary to measure a Ca2+ concentration in a neuron. Observers acquire fluorescence images of fluorescent dyes introduced in neurons. Then they can evaluate intensity of the fluorescence images which respond to changes in the Ca2+ concentration. Thus neural activity is examined by the fluorescence images of the neuron. However, observing the specified neuron in C. elegans for a long time is very difficult since a head of C. elegans moves quickly. To solve this problem, we develop a microscope system which can track a particular region of C. elegans and monitor fluorescence emitted by fluorescent protein in C. elegans. In experimental results, we show that the microscope system can track the head region of moving C. elegans and monitor fluorescence emitted by a chemosensory neuron ASER at 20x magnification.