This paper describes a system named Rowma that can form a network of ROS-based robots. Rowma can construct arbitrary networks among the unspecified number of robots and smartphones according to tasks. In these networks, each node is able to execute arbitrary ROS commands and send/receive arbitrary topics. The most important feature of Rowma is the loose coupling among ROS-based robots that are Rowma clients, and each ROS-based robot works independently without concerning what other robots behave. Specifically, Rowma provides the following functions; sending and receiving arbitrary topics according to the tasks, and execution nodes among robots. Robots can easily join a network because Rowma can be easily integrated into the existing ROS-based system without affecting it. Also, a developer can easily develop robot applications since we distribute Rowma SDKs. These applications can connect to the Rowma network likewise robots do, also, it allows users to develop more intricate applications. This paper describes one of the use cases of Rowma, and show the results of measuring the data transmission delay time using Rowma in two cases.