The present disclosure relates to an apparatus for preparing germane gas and a method for preparing monogermane gas using same. More particularly, the present disclosure relates to an apparatus for preparing germane gas, capable of stably producing a large amount of monogermane gas by mixing starting materials in short time and removing reaction heat at the same time using a reactor having a microstructured channel, and a method for preparing monogermane gas using same. In accordance with the present disclosure, it is easy to control rapid increase of reaction temperature and pressure during mass production of germane gas. Accordingly, monogermane gas can be produced in large scale with high yield.