Homeostasis is maintained by neural reflex controls, which attempt to maintain the body's internal organs within a predetermined range. However, when deviations are significant, homeostasis may not be restored by these standard controls. Therefore, a function called allostasis is necessary to maximize the energy efficiency of bodily functions, anticipate changing needs (e.g., a future increase in body temperature or oxygen demand), and regulate the body's state before dyshomeostasis occurs. In terms of the free energy principle (FEP), the expectation to survive (i.e., to stay within established confines and to maintain homeostatic integrity over time) is realized by a hierarchical prediction mechanism. Allostasis requires the integration of exteroceptive, interoceptive, and proprioceptive signals. In this paper, the neural mechanism of allostasis is discussed in terms of the FEP.