Provided is a power generating device which, while having a basic structure of using a rack and a pinion, increases power generating efficiency and has fewer components, has improved durability due to a simplified structure, and can easily be made in smaller sizes. Configuring a guide mechanism using outer walls of a case of a power generating unit as guides and combining the same with rails provided on inner walls of a case of the power generating device distributes unnecessary stress on the power generating unit over the inner walls of the case to obtain effects of achieving an overall small size, reduction in the number of components, and improved durability.