The ROVs with thin cable system have been developed in JAMSTEC. The thin cable system is very convenient for AUVs, therefore it has been applied for the AUVs that have been developed in JAMSTEC. Conventional ROV systems need the thick cable to supply electric power from mother ship to under-sea vehicle. In the thin cable system, a thick cable is not necessary because a power source like a battery is installed in the vehicle. The communication between the vehicle and mother ship just needs to include observation data from vehicle and control data from mother ship. The data are communicated through only one optical fiber cable that is about 1 mm in diameter. The thin cable system is called "UROV system" in JAMSTEC. UROV system has some merits because of thin cable. First, since the optical fiber cable with the length of 10 km is installed in the small cylindrical case called "spooler", the onboard system is very compact against conventional ROVs. Second, if a tether cable gets entangled in something like under-sea structure, it is considered that a vehicle can not be recovered in worst case. In the case of UROV system, the vehicle can be recovered because it has safety sequence that will release the ballast for surfacing. UROV system has other merits. For example, there is no cable drag. UROV system is available not only ROV but also AUV. URASHIMA developed by JAMSTEC has 3 operation modes. UROV mode is one of these operation modes. This mode is mainly useful for debugging in early stage of developing. When troubles happen, it is difficult for normal AUV that has no cable to know the detailed state on real-time. In that case, in URASHIMA, UROV mode is very convenient debugging of software and hardware. Furthermore, URASHIMA can survey like ROV by optical communication for UROV mode, for example, to watch the TV image on real-time. This paper describes UROV system for 7,000 m class UROV, UROV7K, and AUV, URASHIMA.