Works in the large area such as farm work and inspection by conventional autonomous mobile robot takes a long time to complete, as the power supply is limited and affected by environment. We have been developing a far-reach tethered working tool between mobile platforms to solve such problems. The main components of the system are the following: a tethered tool, two mobile platforms, two wire winding mechanism (winches). In this paper, we propose grass cutting system in the large area as an application by the far-reach tethered working tool. Such system presents great advantages over existing devices as they are capable of efficient work in uneven terrain and ensuring the safety of the work. We show design of the tethered tool with the grass cutting device, which moves along the undulating of the ground surface. After addressing important design concept, we present a prototype model of the tethered tool for grass cutting. The tethered tool moves based on the motion planning using virtual impedance method and the distance information measured by the ToF ranging sensor. Laboratory-based tests were conducted to verify the effectiveness of the system. Experiment results showed that the tethered tool moved along the undulating of the floor and avoided obstacles.