This paper presents a routing problem of multiple mobile robots collaborating with human workers in a ware-house. In a scenario considered in this paper, the warehouse stores several different of heavy items on multiple stock locations. The human worker pick items up from corresponding stock location, puts them on one of the mobile robot carriers, and then visits another stock location. The multiple mobile robots deployed in the warehouse visit stock locations by following specified routes, carry several items for the worker, and deliver them to a dispatch area in the warehouse. Here, an appropriate number of robots, the maximum speed of each robot, and the load capacity of each robot should be quantitatively analyzed while the routing problem of the multiple robots with the human worker in the warehouse is considered. The research in this paper develops a simulation framework in which the worker and the robots cooperatively performs the pickup and dispatch tasks. An algorithm of the routing problem in the framework is addressed based on the Mixed-integer linear programming, which calculates the order of the picking points for each robot. Simulation studies for an actual warehouse are performed with different conditions between the number of robots, speed, and load capacity. The simulation framework calculates a task time which indicates how long the robots and worker need to complete one shipping list. The task time also quantitatively indicate how large the multiple robots reduce the work load of the human worker.