In this paper, we propose a new model for describing software processes and an estimation method for the quality, cost and delivery date of a software project. The model was developed based on the experience and the data from the software development process at OMRON corporation. The model consists of a Project model and a Process model. The Project model focuses on three key components: activity, product and developer of the project. Process model includes a set of Activity models, each of which specifies design, coding, review, test, and debug activities respectively using GSPN(Generalized Stochastic Petri-Net). The new model can take the influence of human factors into account by introducing the concept of “workload”. Next, we develop a simulator which supports description of the process and executes the process described by the Activity model. As the result of its execution, we get the estimated values for the quality, cost and delivery date of the target process. Then, we apply the simulator to real software processes at OMRON corporation and compare these estimated values with actual data. The experimental results show the applicability of the proposed simulator to improve real software process.