Disclosed herein is a module housing of a unit module including battery cells, the module housing including a first cover member and a second cover member coupled to each other for covering entire outer surfaces of the battery cells, mounting grooves formed at an inside end of at least one of the first and second cover members such that the battery cells are mounted in the respective mounting grooves, and an injection port formed at the module housing such that a thermoplastic resin is injected to interfaces between the mounting grooves and the battery cells through the injection port in a state in which the battery cells are mounted in the module housing.