Intermittent and chaotic motions of the hydrogen bond network are studied with the molecular dynamics (MD) simulation. By analyzing the fluctuation in the radial distribution function, it is shown that individual water molecule alternately goes through two different periods; the structured period during which the local structure around the molecule is developed more than the average and the destructured period during which the local structure is less developed. At room temperature the lifetime of each period is hierarchically distributed from a few hundreds fsecs to several psecs. This intermittent structural fluctuation is quantitatively analyzed by defining a new quantity, local structure index (LSI). Molecules which have the large LSI value have tendency to be close to each other to form clusters. Temporal and spatial correlations of the structural order are studied with this new method. The analyses of the structural fluctuation provide a new perspective to study the collective motion of water molecules.