The most important item in the design of an engine room construction or a pump room construction of a tanker is to pay attention to the machinery installed there. Usually, the hull structure is to be designed based on stress not on deflection. However, in the design of engine room construction of a large ship, meticulous care must be taken on the deflection to prevent machinery troubles. The engine room is prone to vibration problems, located adjacent to main vibration exciting sources such as the main engine and the propeller, and therefore, the structural rigidity is also important for vibration prevention. In this chapter, various issues on the design of engine room construction are explained, including steel wall arrangement in engine room and superstructure, turbine flat deflection of a tanker, double bottom deflection causing damage to shaft bearings and reduction gear. Especially, regarding the double bottom deflection, detailed explanations are made with respect to its mechanism, allowable limit, and how to control it to prevent shaft alignment problems.