Software cost estimation model is used to subcontract or plan for the software development project. Overestimation can waste resources, such as human power, money and time. On the other hand, underestimation can lower the quality of software and cause budget overrun and schedule slippage. Although the practitioners of most domains need the software cost estimation model, the practitioners of the defense domain is more concerned with this model because the budget for software development is very large. In order that the software cost estimation model is specialized in the defense domain, it should consider the characteristics of this domain. However, the existing generic models, such as COCOMO II, Price-S, and SEER-SEM, do not consider the characteristics of the defense domain. So, we develop the software cost estimation that are specialized for the defense domain and suggest the activities that are performed for developing this model. These activities contain model development process definition, defense domain analysis, cost factor identification, data collection, and CER development. We compare our model with COCOMO II which is the well-known generic model. We also examine the effect of the identified five factors through experiment. The result shows that the accuracy of our model is superior to the accuracy of COCOMO II and the five factors well characterize the defense domain. This model can be used to prevent the acquisition organization from wasting the budget and it also support the project management by allocating the appropriate resources.