Many mobile robots using undulatory locomotion have been developed for search or rescue operations in narrow or dangerous places where people cannot enter, because undulatory locomotion can be employed for locomotion in various environments. However, an environmental change will require a mobile robot to change the pattern of undulatory locomotion. Recent studies showed that a leech changes its undulatory locomotion pattern depending on changes in the environment, and this is derived from sensory feedback from the muscle strain to the central nervous system. We propose an adaptive control method for a mobile robot that is inspired by the leech's adaptive locomotion. First, we make clear the role of this adaptive behavior of a leech by using the numerical leech model proposed in the preceding study. After that, we construct the simplified adaptive control method that simulates the leech's adaptive nervous system. Finally, we evaluate the effectiveness of our proposed control method with a numerical model. Our study showed that the proposed adaptive control method achieves regulation of body load due to the fluid despite a change in the fluid viscosity. Moreover, our control method reproduced the adaptive behavior of a living leech to changes in the fluid viscosity.