For research and development for mobile communication schemes and specification designs, a variety of system level simulators suitable for purpose and usage are required. For example, the 5G mobile communication system has three service scenarios for eMBB, mMTC, and URLLC and eight Key Performance Indicators (KPI) to support them. It also has various types of network configurations and channel models with different frequency bands. Therefore, a simulator for verifying such various systems require the following two features. First, it should support the reusability of the simulator. The reusability of the simulator can be supported through the library of functions that are used repeatedly. When the simulator's library is designed with modular concept, the user of the system level simulator can construct their simulator in an efficient way. Second, it should support the flexibility of using simulators. The simulator should be designed to be intuitive so that users can expect flexible use. This requires documentation on the simulator and readability of the completed code. It can also support by modular design. To satisfy these requirements, we will develop open, modular and flexible simulator with hierarchical structures and modular design and we will also verify various 5G mobile communication systems by the simulator.